BIOLOGY 

CONN 


LIBRARY 
G 


BIOLOGY 

AN  INTRODUCTORY  STUDY 


FOR  USE  IN  COLLEGES 


BY 

HERBERT  W.  CONN,  PH.D. 

I  ' 
PROFESSOR  OF  BIOLOGY  IN  WESLEYAN   UNIVERSITY 


SILVER,  BURDETT  AND  COMPANY 

BOSTON  NEW  YORK  CHICAGO 


BIOtOfiY 


COPYRIGHT,  1912,  BY 
SILVER,  BURDETT  AND  COMPANY 


PREFACE 

THIS  work  is  intended  to  serve  as  an  introduction  to  the 
study  of  botany  and  zoology.  It  has  been  for  some  time  recog- 
nized that  there  is  a  series  of  laws  and  principles  which  relate 
both  to  animal  and  plant  life,  and  another  series  of  important 
facts  which  refer  to  the  relations  of  animals  and  plants  to 
each  other.  In  helping  to  a  comprehension  of  nature,  these 
interrelations  are  really  of  more  significance  than  the  detailed 
study  of  certain  animals  and  plants.  But  with  the  tendency 
shown  frequently  in  our  educational  system,  to  divide  biology 
into  zoology  and  botany,  there  is  danger  that  these  fundamental 
truths  and  interrelations  be  neglected,  since  a  consideration  of 
them  belongs  strictly  neither  to  zoology  nor  to  botany. 

To  students  of  the  age  of  those  in  secondary  schools,  the 
study  of  such  concrete  facts  as  the  description  of  animals  and 
plants  is  most  attractive;  and  for  them,  courses  in  elementary 
botany  and  zoology  are  eminently  appropriate.  But  to  students 
of  the  greater  maturity  of  college  grade,  the  study  of  the  funda- 
mental biological  laws  is  more  stimulating  and  better  calculated 
to  develop  the  thinking  powers.  It  is,  therefore,  the  author's 
belief  that  the  proper  way  for  older  students  to  begin  the 
study  of  the  great  department  of  biology  is  to  consider  the 
fundamental  principles  relating  to  both  animals  and  plants, 
before  either  of  these  groups  is  studied  in  detail.  After  the 
student  turns  his  attention  more  particularly  to  zoology  or 
botany,  he  is  likely  to  be  engrossed  in  the  details  of  the  life 
and  structure  of  animals  and  plants,  and  so  almost  inevi- 
tably neglects  the  broader  fundamental  laws  which  should 
correlate  the  phenomena  of  life  as  one  science.  Unless,  there- 
fore, the  foundation  principles  of  biology  be  studied  as  an 
introductory  course,  it  is  very  probable  that  they  will  be 
neglected.  For  this  reason,  this  text  has  been  provided  as 

iii 

544529 


ir  PREFACE 

an  introductory  survey  of  the  laws  which  apply  to  both  ani- 
mals and  plants,  and  those  principles  which  coordinate  and 
correlate  them.  It  is  hoped  that  it  may  have  some  influence 
in  developing  the  study  of  the  fundamental  principles  of  biology 
as  an  introductory  course,  thus  supplanting  the  old  custom  of 
plunging  the  student  at  the  outset  more  specifically  into  zoology 
or  botany. 

It  is  designed  that  this  work  shall  be  an  elementary  study 
of  biology,  on  a  par  with,  and  parallel  to,  elementary  physics 
and  chemistry.  Logically  it  should  follow,  rather  than  pre- 
cede, these  two  sciences,  although  it  may  be  taken  simultane- 
ously with  them.  Its  place  in  a  curriculum  should,  therefore, 
be  about  the  same  as  that  of  elementary  physics  and  chemistry; 
and  as  developed  in  the  following  pages,  it  belongs  to  the  be- 
ginning of  college  work. 

In  preparing  these  pages,  it  has  been  recognized  fully  that 
a  certain  amount  of  laboratory  work  is  necessary  in  order  that 
the  student  may  properly  understand  biological  phenomena. 
It  is  also  appreciated  that,  with  the  present  development  of 
the  teaching  of  biology  and  the  present  equipment  of  many  of 
our  institutions,  it  is  frequently  impossible  to  introduce  any 
extended  laboratory  work,  on  account  both  of  insufficient  equip- 
ment and  lack  of  time  in  the  already  crowded  courses  of  study. 
For  this  reason,  the  chapters  have  been  arranged  so  that,  where 
necessary,  they  can  be  used  without  the  accompanying  labora- 
tory demonstrations.  Although  this  is  an  undesirable  method 
of  studying  biology,  the  author  believes  that  the  biological 
principles  covered  in  the  following  pages  may  be  comprehended 
in  a  fairly  satisfactory  manner,  even  though  the  student  does 
not  have  the  opportunity  of  making  the  laboratory  tests.  It 
is  hardly  necessary  to  state,  however,  that  as  much  practical 
laboratory  work  as  possible  should  accompany  the  study  of 
the  text.  For  this  reason,  outlines  of  the  correlative  laboratory 
work  have  been  added  at  the  end  of  the  chapters.  In  all  cases 
where  laboratory  work  is  possible,  students  should  be  required 


PREFACE  7 

to  make  careful  drawings  of  the  objects  studied.  Wherever 
time  permits,  the  laboratory  work  outlined  here  should  be 
expanded  by  instructors.  (For  more  detailed  laboratory  di- 
rections than  can  be  given  here,  reference  should  be  made  to 
the  many  excellent  handbooks  of  zoological  and  botanical  lab- 
oratory work,  a  few  of  which  are  mentioned  in  the  brief  bibli- 
ographies at  the  close  of  the  chapters.) 

In  place  of  the  ordinary  index  there  will  be  found  at  the 
close  of  the  book  a  glossary-index.  In  it  are  given  brief  defi- 
nitions of  all  the  technical  words  used  in  the  book,  with  deri- 
vations and  with  page  references.  To  make  this  more  valuable 
as  a  reference  glossary,  some  common  biological  words  which 
do  not  chance  to  be  used  in  the  text  are  defined.  These  are 
easily  recognized  from  the  fact  that  they  have  no  page  numbers. 

H.  W.  CONN. 


CONTENTS 

APTER  PAGE 

I.  THE  SCOPE  OF  BIOLOGY  .  .  .  .  .  1 
The  New  Biology  and  the  Old.  The  Funda- 
mental  Properties  of  Living  Things.  Chemical 
Composition  of  giving  Tissues.  Origin  of 
Life.  The  Biological  Sciences:  Morphology. 
Physiology.  Zoology  and  Botany. 

II.  CELLS  AND  THE  CELL  THEORY  .  .  ,  .  26 
Organisms.  The  Cell  as  the  Unit  of  Organic 
Structure.  Cell  Structure.  Cell  Substance  or 
Protoplasm.  The  Nucleus.  The  Centrosome. 
The  Cell  Wall.  Cell  Functions.  History  of 
the  Cell  Doctrine:  1.  The  Early  Conception 
of  the  Cell  (1839-1861).  2.  Protoplasm  and 
the  Mechanical  Theory  (1861-1885).  3.  The 
Nucleus  and  its  Significance  (1880  to  the 
present).  What  is  Meant  by  Protoplasm. 

III.  UNICELLULAR  ORGANISMS 52 

Animals:  AmcebcL  Paramedwn.  Plasmo- 
dium  Malarice.  Chilomonas.  Pandorina.  In- 
termediate Organisms:  Peranema.  Euglena. 
Plants:  Pleurococcus.  Saccharomyces — Yeast. 
•  Bacteria. 

IV.  CELL    MULTIPLICATION    AND    THE    CELLULAR 

STRUCTURE  OF  ORGANISMS     ....       85 
Cell    Division    or   Karyokinesis.      Unicellular 
and  Multicellular  Organisms.     Penitillium,  a 
Simple  Multicellular  Plant.     Other  Species  of 
Molds. 

vii 


viii  CONTENTS 

CHAPTER  PAGE 

V.     THE   CASTOR  BEAN,  A  COMPLEX  MULTICELLU- 

LAR  PLANT        .  .  .103 

The  Castor  Bean  (Ricinus  Communis).  Gross 
Structure.  Structure  of  the  Stem.  Structure 
of  the  Root.  Structure  of  the  Leaf.  Repro- 
ductive Organs. 

VI.    THE  PHYSIOLOGY  OF  A  TYPICAL  PLANT     .       .126 
Photosynthesis  or  Starch  Manufacture.    Me- 
tastasis.   Photosynthesis  and  Metastasis  Con- 
trasted.     Miscellaneous    Functions    of    Plant 
Life. 

A     VII.      MULTICELLULAR   ANIMALS!      HYDRA  FuSCA  .          .       138 

General  Life  Functions  of  Animals.  Animal 
Biology.  Hydra  Fusca,  a  Simple  Multicellu- 
lar  Animal.  The  Relation  of  the  Whole  Or- 
ganism to  its  Different  Parts. 

VIII.     MULTICELLULAR  ANIMALS:      THE   EARTHWORM 

(Lumbricus) 155 

Anatomy.  Microscopic  Anatomy,  orJEttstology. 

X     IX.     MULTICELLULAR  ANIMALS:    THE  FROG  (Rana). 

GENERAL  DESCRIPTION 175 

X.     THE  PHYSIOLOGY  OF  AN  ANIMAL         .       .  t    .     204 
Physiology  of  the  Earthworm. 

L  THE  DIFFERENCES  BETWEEN  ANIMALS  AND 
PLANTS:  THE  MUTUAL  RELATIONS  OF  OR- 
GANISMS   217 

The  Differences  between  Animals  and  Plants. 
Contrast  between  the  Activities  of  Animals 
and  Plants.  The  Mutual  Relations  of  Organ- 
isms. Nature's  Life  Cycle. 


CONTENTS  ix 

PAGH 

REPRODUCTION  :  SEXUAL  AND  ASEXUAL  METHODS    238 

General  Types  of  Reproduction.  Reproduction 
in  Unicellular  Organisms.  Reproduction  in 
Multicellular  Organisms.  Division  without 
Cell  Union.  Multiplication  by  Cell  Union. 
The  Union  of  the  Sex  Bodies  or  Fertilization. 
The  Relation  of  the  Chromatin  to  Heredity. 
The  Purpose  of  the  Union  of  the  Sexes. 


DISTRIBUTION     OF     SEXUAL    AND    ASEXUAL 

METHODS.    ALTERNATION  OF  GENERATIONS    262 

Summary  of  the  Methods  of  Reproduction. 
Origin  of  Sex  Union.  Distribution  of  Asexual 
Reproduction.  Distribution  of  Sexual  Repro- 
duction. Reproductive  Bodies  or  Reproductive 
Cells.  Cross  Fertilization  the  Rule.  Alterna- 
tion of  Sexual  with  Asexual  Methods  of  Repro- 
duction. 

XIV.    DEVELOPMENT  OF  THE  FERTILIZED  EGG     .       .     280 

Embryology  and  Metamorphosis.  Embryology 
of  the  Frog. 

XV.    THE  SOURCE  AND  NATURE  OF  VITAL  ENERGY  .     292 

Matter  and  Energy.  The  Conservation  of 
Energy.  The  Transformation  of  Energy.  The 
Living  Organism  as  a  Machine.  The  Life  of  a 
Plant.  The  Life  of  an  Animal. 

XVI.    THE  MECHANICS  OF  THE  LIVING  MACHINE       .     303 

Details  of  the  Action  of  the  Machine.  Vital 
Force  or  Vitality.  Summary.  What  is  Life? 


x  CONTENTS 

CHAPTER  PAGE 

XVII.  THE  ORIGIN  AND  DEVELOPMENT  OF  ORGANISMS: 

HEREDITY  AND  VARIATION"     .        .  .     325 

The  Origin  of  the  Living  Machine  Not  Ex- 
plained. The  Forces  Which  Have  Produced 
Organisms.  Conformity  to  Type.  Divergence 
from  Type. 

XVIII.  THE  ORIGIN  OF  THE  LIVING  MACHINE:  ADAP- 

TATION; THE  FORCES  OF  ORGANIC  EVOLU- 
TION      342 

Adaptation.     The  Theory  of  Evolution. 

XIX.     CLASSIFICATION  AND  DISTRIBUTION     .       .       .     364 

Classification  (Taxonomy) .  The  Significance 
of  Classification.  An  Outline  of  the  Classi- 
fication of  the  Living  World.  Distribution 
of  Animals  in  Space  and  Time.  Distribution 
of  Organisms  in  Time:  Paleontology. 


GLOSSARY-INDEX 


387 


BIOLOQY 

CHAPTER   I 
THE  SCOPE  OF  BIOLOGY 

THE  NEW  BIOLOGY  AND  THE  OLD 

BIOLOGY  is  often  described  as  the  most  recent  of  the  sciences, 
despite  the  fact  that  it  was  one  of  the  first  to  be  studied.  Four 
centuries  before  Christ,  animals  were  dissected  and  described 
by  Aristotle,  and  from  that  time  on,  the  study  of  living  things 
has  never  ceased.  In  the  last  half  century,  however,  the  study 
of  vital  phenomena  has  assumed  a  new  aspect.  Formerly 
animals  and  plants  were  studied  only  as  objects  to  be  classified 
and  named;  now  they  are  studied  as  objects  to  be  explained. 

Progress  of  Scientific  Thought. — This  new  method  of  bi- 
ological study  is  only  another  expression  of  man's  changed 
attitude  toward  all  natural  phenomena.  In  early  times,  people 
imagined  that  all  the  phenomena  of  nature  which  they  could 
not  understand  were  produced  by  gods.  One  god  caused  the 
winds;  another  the  motions  of  the  sun  and  stars.  Gradually 
these  conceptions  have  been  changed  by  the  attitude  of  modern 
science.  First,  the  motions  of  the  heavenly  bodies  were  ex- 
plained under  the  general  law  of  gravitation.  Then,  the  mys- 
terious phenomena  of  fire  and  of  electricity  were  comprehended 
under  the  laws  of  chemistry  and  physics.  Later,  the  various 
changes  on  the  earth's  surface,  such  as  the  formation  of  moun- 
tains, of  valleys,  of  rivers,  and  of  plains,  were  explained  as  the 
result  of  the  ordinary  forces  of  nature. 

In  all  this  there  has  been  a  progress  in  one  direction,  namely, 
toward  the  explanation  of  natural  phenomena  by  natural 
forces.  The  most  recent  of  the  natural  phenomena  to  be 
studied  with  this  end  in  view,  are  those  associated  with  living 

1 


2  BIOLOGY 

animals  and  plants.  The  question  whether  the  activities  of 
animals  and  plants  can  be  explained  by  the  same  forces  found 
elsewhere  in  nature,  and  the  attempt  to  answer  this  question 
in  the  affirmative,  form  the  basis  of  the  new  science  of  biology. 
Modern  biology  is  thus  something  more  than  the  study  of 
animals  and  plants  as  dead  objects  to  be  collected,  named, 
and  classified.  It  is  a  study  of  animals  and  plants  in  action; 
as  living  beings  to  be  related  to  their  environment.  It  is  this 
attempt  to  explain  life  processes  which  may  be  said  to  have 
raised  biology  to  the  rank  of  a  new  science. 

THE  FUNDAMENTAL  PROPERTIES  OF  LIVING  THINGS 

Distinction  between  the  Living  and  the  non-Living. — Since 
biology  (Gr.  bios  =  \ife -\-logos  =  discourse)  is  the  science  of  living 
things,  we  must  first  ask  how  living  things  may  be  distinguished 
from  non-living.  While  it  is  a  comparatively  easy  matter  to 
recognize  the  distinction,  it  is  difficult  to  draw  it  sharply. 
Indeed,  some  biologists  are  of  the  opinion  that  no  rigid  line 
can  be  drawn,  and  that  there  are  some  states  of  matter  which 
are  halfway  between  the  living  and  the  non-living.  Whether  or 
not  this  be  so,  it  certainly  is  true  that  between  most  forms 
of  matter  which  we  call  alive  and  those  which  we  call  non- 
living, there  is  a  marked  and  recognizable  difference,  although 
it  may  be  difficult  to  define  it  accurately.  Four  or  five  fun- 
damental properties  are  characteristic  of  life: 

1.  Activity. — The  most  noticeable  difference  between  the 
living  and  the  non-living  is  in  the  presence  or  absence  of  spon- 
taneous activity.  If  we  wish  to  find  out  whether  any  given 
body  is  alive,  we  watch  it  carefully  to  see  if  it  shows  any  power 
of  independent  activity,  and  if  it  does  so,  we  call  it  alive.  If 
the  object,  a  seed  for  example,  seems  to  be  perfectly  dormant, 
we  may  put  it  under  conditions  in  which,  if  alive,  it  will 
develop  activity.  If  it  then  begins  to  grow  into  a  plant  we  say 
that  the  seed  was  alive  at  first  but  dormant.  If,  however,  it 
fails  to  show  any  power  of  developing  into  a  plant  when  placed 


THE  SCOPE  OF  BIOLOGY  3 

in  proper  conditions,  we  conclude  that  the  seed  is  not  alive. 
Hence  the  best  criterion  that  we  have  for  separating  the  living 
from  the  non-living  is  to  determine  whether  or  not  the  body 
in  question  either  shows  any  signs  of  independent  activity  or, 
when  put  under  proper  conditions,  may  be  made  to  show  any 
signs  of  such  activity. 

Automatic  activity. — The  simple  fact  of  showing  activity  is, 
however,  not  enough  to  serve  as  a  criterion  of  life.  Other 
things  besides  living  beings  have  the  power  of  activity.  A 
watch,  or  a  locomotive,  or  a  steam  engine  certainly  shows 
activity,  and  yet  none  of  these  is  alive.  There  is,  however, 
one  distinction  between  the  activity  of  such  machines  and 
the  activity  of  a  living  organism.  Machines  show  activity 
only  when  they  are  started  into  action  by  some  outside  in- 
fluence; while  a  living  organism  develops  activity  from  its 
own  internal,  independent  power.  With  this  modification, 
the  first  criterion  that  we  have  for  distinguishing  the  living 
from  the  non-living  is  the  power  of  developing  automatic 
activity,  and  only  objects  possessing  this  power  do  we  speak 
of  as  being  alive. 

2.  Death. — The  fact  that  living  things  show  automatic  activ- 
ity has  a  converse  side.  This  activity  may  cease,  the  object- 
losing  its  power  of  showing  spontaneous  activity.  This  consti- 
tutes the  phenomenon  spoken  of  as  death.  To  define  either 
life  or  death  has  proved  a  puzzle  to  both  science  and  philosophy. 
For  our  purpose,  however,  they  can  be  fairly  well  defined  as 
follows:  By  life,  we  mean  the  possession  of  the  power  of  show- 
ing spontaneous,  automatic  activity;  by  death,  we  mean  the 
disappearance  of  this  power.  Why  an  animal  or  plant,  when  it 
dies,  loses  this  power,  we  do  not  know.  In  some  cases  it  is 
undoubtedly  because  the  complicated  machinery  which  com- 
poses the  body  is  injured  and  consequently  cannot  work 
properly.  This  we  find  true  also  in  the  case  of  ordinary  ma- 
chines. If  a  locomotive  should  burst  its  cylinders,  it  would  no 
longer  be  able  to  run.  If  a  watch  has  its  mainspring  broken, 


4  BIOLOGY 

it  is  thrown  out  of  adjustment  and  consequently  does  not 
show  activity.  So  in  regard  to  living  things;  the  inability  to 
show  further  activity  may  undoubtedly  be  attributed  to  the 
fact  that  the  machinery  is  out  of  order.  If,  for  example,  the 
beating  of  the  heart  ceases  for  any  length  of  time,  life  activity 
must  cease,  because  life  activity  is  dependent  on  the  circulation 
of  the  blood.  Thus,  in  many  cases  we  know  positively  that 
death  comes  from  the  breaking  down  of  the  machine.  Whether 
death  means  anything  more  than  the  breaking  down  of  the 
machine;  whether  anything  is  lost  which  can  be  called  the 
life  force,  is  one  of  the  questions  over  which  philosophy  and 
biology  have  puzzled  for  long  years,  and  upon  which  they 
have  not  reached  any  definite  conclusion. 

3.  Growth. — All  organisms  disintegrate  by  oxidation  and 
waste.  When  a  piece  of  wood  reaches  the  required  temperature 
to  unite  with  the  oxygen  of  the  air,  it  burns.  Waste  products 
appear  as  gases  and  ashes,  and  the  wood  disappears.  In  a 
similar  way,  by  union  with  oxygen  the  living  body  is  being 
constantly  converted  into  waste  products  which  are  given  off 
from  the  body  as  excretions.  As  a  result  the  organism  is 
constantly  disintegrating.  This  would  inevitably  result  in  the 
disappearance  of  the  organism  if  it  were  not  for  the  opposite 
power  of  reintegration,  or  growth. 

All  living  things  have  the  power  of  growing,  and  no  object 
that  is  not  alive  has  this  power.  It  is  true  that,  under  some 
circumstances,  crystals  may  increase  in  size,  and  this  is  some- 
times referred  to  as  a  growth  of  the  crystals;  but  it  is  a  totally 
different  kind  of  growth  from  that  which  we  find  in  living 
things.  In  the  case  of  the  crystal,  the  new  material  is  simply 
laid  upon  the  outside  of  the  old,  layer  after  layer,  and  the 
apparent  growth  is  really  an  increase  in  size,  by  the  process  of 
accretion.  In  the  growth  of  the  living  organism,  material  is 
taken  inside  of  the  body,  and  there  it  is  transformed  into 
compounds  like  those  of  the  living  organism  which  has  ab- 
sorbed it.  Thus  the  living  organism  increases  from  within, — 


THE  SCOPE  OF  BIOLOGY  5 

a  type  of  growth  spoken  of  as  intussusception  (Lat.  intus  = 
within  +  suscipere  =  to  take  up).  With  this  understanding  of 
growth  we  can  state  that  nothing  grows  except  living  things. 
As  the  result  of  their  activities,  living  things  are  constantly 
wasting  away;  but  by  growth  they  repair  and  keep  pace  with 
their  own  wastes  and  remain  in  a  practically  constant  condi- 
tion, in  spite  of  their  ceaseless  activity.  In  time,  however, 
the  disintegrating  tendencies  surpass  the  powers  of  repair,  and 
the  organism  dies  of  old  age. 

4.  Reproduction. — The  power  of  reproduction  is  found  only 
in  the  realm  of  the  animate  world,  for  only  a  living  organism 
can    produce  another  like   itself.      Inanimate  things  cannot 
reproduce  their  kind. 

As  a  result  of  this  power  of  reproduction,  held  in  common 
by  all  things  possessed  of  life,  there  is  a  constant  replacement 
of  the  individual,  a  constant  wearing  out  and  death,  a  constant 
rebirth  and  growth,  the  new  organism  ever  replacing  the  old  as 
it  disintegrates  and  disappears.  There  is  a  constant  tendency 
to  undergo  cyclical  changes  present  in  all  manifestations  of  life. 

5.  Consciousness. — Consciousness  is  characteristic  of  some 
living  bodies,  but  is  probably  not  universal  among  them,  for  it 
is  practically  certain  that  life  occurs  in  many  places  without 
consciousness,    although   some    theorists   have   endeavored   to 
argue  that  all  forms  of  life,  even  the  plants,  have  a  very  dim 
form  of  consciousness.    This  is  very  doubtful,  and  we  cannot 
regard    consciousness    as    universally    characteristic    of    life. 
Wherever  consciousness  is  found,   however,   it  indicates  the 
presence  of  life,  and  thus  may  be  deemed  one  of  the  most 
important  signs  of  life. 

CHEMICAL  COMPOSITION  OF  LIVING  TISSUES 

Chemical  Elements  in  Living  Tissues. — Although  there  is  a 
large  variety  of  chemical  compounds  found  in  living  animals  and 
plants,  nevertheless  there  is  a  certain  uniformity  among  them. 
All  animals  and  plants  are  made  up  primarily  of  a  small  num- 


BIOLOGY 


her  of  elements,  nine  chemical  elements  being  ordinarily  pres- 
ent in  living  things,  four  of  which  predominate,  while  the  other 
four  are  present  only  in  small  quantities.  They  are  as  follows :  — 

Oxygen,  a  colorless,  odorless  gas,  forming  about  one-fifth 
of  the  atmosphere. 

Carbon,  a  solid  at  ordinary  temperatures.  Charcoal,  graphite, 
lampblack,  and  diamond  are  examples  of  almost  pure  carbon. 

Hydrogen,  a  gas,  the  lightest  of  all  known  substances  and 
highly  inflammable. 

Nitrogen,  a  colorless,  odorless  gas  which  comprises  about 
four-fifths  of  the  atmosphere. 

Sulphur,  phosphorus,  calcium,  iron,  and  potassium  consti- 
tute the  other  chemical  elements  that  are  found  in  living 


Oxt/ye/j 


Mitroq 


tfydroqen 


FIG.  1. —  DIAGRAM  SHOWING  THE  RELATIVE  PROPORTIONS 

OF  THE  CHIEF  ELEMENTS  MAKING  UP  A  LIVING  BODY 

things.  Only  very  small  amounts  of  these  elements  are  present, 
although  calcium  is  found  in  animals  in  considerable  quantities 
in  the  bone.  Figure  1  shows  diagrammatically  the  relative 


THE  SCOPE  OF  BIOLOGY  7 

proportions  of  the  chief  chemical  elements  in  the  animal  body. 
Oxygen,  carbon,  hydrogen,  and  nitrogen  constitute  about 
98  per  cent  of  the  animal  body  and  not  far  from  the  same  pro- 
portion of  the  composition  of  the  body  of  most  plants.  These 
four  elements  also  constitute  by  far  the  largest  proportion  of 
the  material  present  in  the  earth's  crust;  so  that  the  living 
body  is  made  of  the  same  materials  that  are  most  abundantly 
present  in  the  inanimate  world  around  us. 

Chemical  Compounds  in  Living  Tissues. — It  is  perfectly 
evident  that  the  elements  enumerated  do  not  exist  in  the 
living  body  as  uncombined  elements.  Two  or  more  of  them 
are  always  united  as  chemical  compounds  to  form  a  substance 
different  from  either  of  them.  The  chemical  compounds  that 
are  present  in  the  bodies  of  animals  and  plants  are  of  an  endless 
variety;  but  a  few  general  types  are  most  widely  present  and 
may  be  regarded  as  the  fundamental  compounds  of  living 
things.  These  compounds  are  important,  since  they  enter 
into  the  food  of  all  animals.  They  are  as  follows:  proteids, 
carbohydrates,  fats. 

Proteids. — Proteids  are  extremely  complex  substances,  com- 
posed chiefly  of  the  elements:  carbon,  oxygen,  hydrogen,  and 
nitrogen,  but  containing  also  in  small  proportions  sulphur 
and  the  other  elements  that  have  been  enumerated  above. 
They  are  by  far  the  most  complex  substances  in  living  things; 
that  is,  in  a  proteid  molecule,  there  are  present  more  chemical 
atoms  than  are  found  in  a  molecule  of  any  other  substance 
existing  in  the  animal  body.  The  exact  chemical  composition 
of  proteids  is  not  known  and  it  suffices  for  our  purpose  to 
state,  that  they  are  composed  of  a  highly  complex  combina- 
tion of  the  elements  we  have  mentioned,  so  united  that  hun- 
dreds of  atoms  are  probably  always  combined  to  make  a  mole- 
cule. Some  idea  of  their  complexity  may  be  obtained  from 
the  fact  that  one  chemist  gave  as  a  formula  for  egg-albumen, 
C2o4H322N52O66S2  (a  formula  too  complicated  to  have  any  real 
meaning);  and  indeed,  no  two  chemists  agree  upon  the  chem- 


8  BIOLOGY 

ica!  composition  of  any  proteid.  The  following  are  the  best- 
known  proteids:  albumen,  the  white  of  an  egg;  myosin,  the 
lean  part  of  the  meat;  casein,  the  curd  of  the  milk;  gluten, 
the  sticky  substance  in  flour;  legumen,  a  similar  sticky  material 
present  in  peas  and  beans.  Besides  these,  there  are  many 
other  proteids  present  in  animal  and  plant  tissues.  Living 
tissue  is  almost  entirely  proteid  in  character. 

Sources  of  proteids. —  Since  living  things  are  made  up 
largely  of  proteids,  we  next  inquire  into  the  source  of  these 
proteids.  As  will  be  noticed  later,  green  plants  can  combine 
the  gases  of  the  air  with  the  water  and  certain  minerals  obtained 
from  the  soil,  and  thus  manufacture  their  own  proteids.  Animals 
and  colorless  plants  (fungi)  are  totally  unable  to  manufacture 
proteids  from  inorganic  compounds.  Hence  it  follows  that 
animals  and  the  colorless  plants  depend  upon  the  green  plants 
for  their  proteids,  which  is  simply  another  way  of  stating  the 
fact  that  animals  require  plants  for  their  food.  Although  unable 
to  manufacture  proteids,  colorless  plants  and  animals  are,  how- 
ever, able  to  modify  them  more  or  less,  having  the  power  to 
transform  one  kind  of  proteid  into  another.  If,  for  example, 
an  animal  is  fed  with  the  white  of  an  egg,  it  can  transform 
this  proteid  into  the  proteid  of  muscle,  thus  changing  albu- 
men into  myosin.  Since  animals  are  unable  to  manufacture 
muscles  from  any  substances  but  proteids,  it  follows  that  they 
are  obliged  to  have  proteids  in  their  diet. 

Carbohydrates. — Starches  and  sugars  are  the  best-known 
examples  of  carbohydrates.  They  are  much  simpler  than 
proteids,  consisting  of  only  three  chemical  elements:  carbon, 
oxygen,  and  hydrogen.  These  elements  are  combined  in  mole- 
cules with  the  following  formulas :  C6Hi0O5  (starch)  and  C6Hi2O6 
(sugar).  There  is  quite  a  large  number  of  starches  and  sugars, 
differing  from  each  other  in  some  respects,  but  these  formulas 
are  typical  of  their  general  nature.  It  will  be  seen  from  the 
formulas  that  the  difference  between  the  molecules  of  starch 
and  sugar  is  in  the  presence,  in  sugar,  of  H^O  in  addition 


THE  SCOPE  OF  BIOLOGY  9 

to  the  group  contained  in  the  starch  molecules.  H^O  is  a 
molecule  of  water;  and  hence  we  say  that  if  a  molecule  of  water 
is  added  to  a  starch  molecule,  it  will  convert  it  into  a  sugar 
molecule.  It  must  not  be  understood,  however,  that  this 
can  be  done  by  simply  adding  water  to  starch,  for  the  two 
will  not  combine.  There  are  methods  (see  page  306),  however, 
by  which  they  can  be  made  to  combine,  and  under  these  cir- 
cumstances starch  can  very  easily  be  converted  into  sugar. 

Among  the  different  types  of  sugars,  there  are  two  of  espe- 
cial importance.  One  of  these  is  grape  sugar,  also  called  glu- 
cose or  dextrose.  These  three  names  are  closely  related,  al- 
though not  exactly  identical.  The  formula  for  these  is  also 
CeH^Oe.  The  other  type  is  cane  sugar,  obtained  from  sugar 
cane  or  the  sugar  beet.  The  formula  for  this  is  C^H^On, 
which,  as  will  be  noticed,  is  nearly,  but  not  quite,  twice  the 
formula  of  the  grape-sugar  molecule.  By  the  addition  of  a 
molecule  of  water  it  is  possible  to  break  a  molecule  of  the  cane 
sugar  into  two  molecules  of  the  grape-sugar  type,  according 
to  the  following  equation:  Ci2H22Oii+H2O  =  2C6Hi206.  This 
is  commonly  spoken  of  as  inverting  the  sugar. 

Sources  of  carbohydrates. — Carbohydrates  come  almost  wholly 
from  the  vegetable  world.  Green  plants  manufacture  starch 
in  their  leaves  by  combining  the  carbon  dioxid  gas  which 
they  absorb  from  the  air  with  the  water  which  they  absorb 
from  the  soil.  This  starch  is  very  easily  converted  into 
sugar  within  the  plant,  and  then  carried  to  various  parts 
where  it  may  be  stored,  either  in  the  form  of  starch  or  sugar. 
It  is  subsequently  used  by  the  plant  as  food,  or,  if  the  plant 
is  consumed  by  animals,  it  serves  as  their  food.  So  far  as 
known,  there  is  no  other  source  of  carbohydrates  in  nature 
besides  the  green  plants,  and  as  all  animals  and  all  plants 
consume  carbohydrates,  it  is  plain  that  the  whole  living  world 
is  dependent  upon  the  green  plants  for  carbohydrates. 

Hydrocarbons  (Fats). — Good  examples  of  fats  are  found  in  but- 
ter, in  mutton  tallow,  in  lard,  in  olive  oil,  etc.,  and  in  many  other 


10  BIOLOGY 

food  products.  Fats  contain  the  three  elements,  carbon,  oxygen, 
and  hydrogen,  in  this  respect  agreeing  with  the  carbohydrates. 
They  are,  however,  considerably  more  complex  than  carbohy- 
drates, a  molecule  of  fat  containing  more  atoms,  as  is  shown  by 
the  formula  C5iHio4O9,  which  represents  a  common  fat.  When 
treated  by  a  simple  chemical  method,  fats  are  broken  up  into  two 
substances,  one  of  which  is  called  glycerine  and  the  other  a  fatty  acid. 

Sources  of  fats.— ¥  at  can  be  manufactured  by  either  ani- 
mals or  plants  out  of  other  foods.  If  an  animal  is  fed  upon 
proteids  or  carbohydrates,  it  can  manufacture  fat  from  them; 
and  plants  are  able  to  make  fat  out  of  the  food  materials 
which  they  absorb  from  the  air  and  water. 

The  table  on  page  11,  which  illustrates  the  composition  of  a 
few  of  our  common  foods,  shows  that  our  ordinary  diet  con- 
tains a  fair  proportion  of  each  of  these  three  foodstuffs.  It 
will  also  be  seen  from  this  table  that  the  largest  proportion 
of  proteids  comes  from  animal  foods,  while  the  largest  pro- 
portion of  carbohydrates  comes  from  plant  foods. 

>/  ORIGIN  OF  LIFE 

Perhaps  no  feature  of  modern  biology  is  more  important 
than  the  acceptance  of  the  theory  that  every  living  thing 
comes  from  a  living  source.  All  living  animals  and  plants 
with  which  we  are  familiar  to-day  have  originated  from  pre- 
viously existing  life.  The  living  animal  comes  from  the  egg 
that  was  produced  by  another  living  animal;  the  plant  comes 
from  a  seed  that  was  produced  by  another  living  plant.  But 
the  question  of  the  primal  origin  of  life  is  sure  to  intrude  itself 
upon  our  minds,  and  we  are  forced  to  ask  whether  living  things 
can  be,  or  ever  have  been  produced  by  any  other  means.  Did 
there  ever  occur,  or  does  there  occur  in  the  world  to-day,  a 
spontaneous  generation  of  life?  In  other  words,  did  a  living 
thing  ever  arise  from  some  source  which  was  not  alive?  So 
far  as  our  knowledge  of  nature  is  concerned,  there  are  no  means 
of  starting  new  life  except  from  previously  existing  life. 


THE  SCOPE  OF  BIOLOGY 


11 


CO  i-<  r-l  C5  i-H  1-H  <N  i-l  CO  t^  00  CO  TjH  IO  CO 

CO        »OiOTt<  i-i 


CO  00 


O^        O5  CO  CO  GO 


CO  10 
CO  00 


SS 

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3   w 

£  ft 


CO  C5  <N  CO  -^  i— I  i— i  CO  CO  CO  CO  t~~  i— I        CO-— iCOCOC5(M'*i— I  >OOi(N 


T^I  I>  00 

CO        iO 


00  iO  t^  O  t^  T-I        I-H  (M  O  t^-  iO  <N  O5  <M  CO  00  t^  CO  »O 

00  CO  00  Oi  (M  i—  i        i—  *  T-H  r-H        CO  t-(        CO  CO  -^  CO 


id 

fe    « 
w   w 


OOCO  COt^  CO        O5       (M 


i— i  os  co      os      i— i 

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e'oS^^  .  .3  .  •     £«§££^;c$     §sx-s 

|-»»8|      ^gss    glllsa^Jji^l 

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'-fl  o  o  o.SS  ^WDS^^  3      rtr^^^^^Soft 
lU^PU^feOW^^UW     W^^O^mpLH^^PQO^D 


12  BIOLOGY 

Spontaneous  Generation  or  Abiogenesis. — This  idea  of  spon- 
taneous generation,  or  abiogenesis  (Gr.  a  =  without  +  bios  = 
life  -f  genesis  =  generation),  has  been  before  the  scientific  world 
for  centuries.  The  ancients  in  the  time  of  Aristotle,  and  for 
centuries  later,  had  no  especial  question  in  regard  to  the  matter, 
and  took  it  for  granted  that  living  things  did  come  from  in- 
animate matter.  Virgil  tells  us  of  bees  coming  from  the  flesh 
of  bullocks;  Ovid  recounts  that  slime  begets  frogs;  and  many 
centuries  afterwards,  we  read  that  water  produces  fishes  and 
that  mice  can  come  from  old  rags.  Although  to-day  these 
ideas  seem  nonsensical,  once  they  appeared  perfectly  logical. 

Experiments  of  Redi. — This  idea  that  life  could  come  from 
non-living  matter  was  held  without  question  during  the  earlier 
centuries,  and  indeed  until  about  the  17th  century.  In  1680 
an  Italian  named  Redi  made  an  observation  which  led  him  to 
what  was  at  that  time  a  rather  startling  conclusion.  It  had  pre- 
viously been  observed  that  fly  maggots  made  their  appearance 
in  decaying  flesh,  and  it  was  taken  for  granted  that  they  devel- 
oped spontaneously.  Redi  noticed  flies  hovering  over  meat, 
and  demonstrated  by  experiments  that  if  the  flies  were  kept 
away  by  simply  tying  paper  over  a  bottle  containing  the  meat, 
maggots  could  never  develop  in  it.  A  little  further  study 
proved  that  the  flies  laid  eggs  on  the  meat  which  developed 
into  fly  maggots.  From  this  observation  he  drew  the  far- 
reaching  conclusion  that  spontaneous  generation  did  not  occur 
and  that  all  living  things  came  from  living  ancestors. 

This  conclusion  started  a  dispute  which  lasted  for  two  cen- 
turies and  was  not  fully  settled  until  about  1875.  For  the 
conclusion  of  Redi,  that  all  living  things  came  from  living 
ancestors,  was  vigorously  disputed  by  the  adherents  of  the  old 
idea  that  life  could  arise  spontaneously.  Many  ingenious 
experiments  were  devised  to  settle  the  question.  It  did  not 
take,  long  to  prove  that  so  far  as  the  larger  animals  and  plants 
were  concerned,  the  conclusion  of  Redi  was  correct.  But  just 
at  this  time  the  newly  invented  microscope  was  beginning  to 


THE  SCOPE  OF  BIOLOGY 


13 


show  a  world  of  invisible  life,  and  in  the  various  bottles  and 
flasks  used  in  these  early  experiments,  a  large  number  of  micro- 
scopic forms  of  life  appeared  in  spite  of  all  attempts  made  to 
prevent  their  entrance.  Although  in  a  piece  of  meat  no  fly 
maggots  developed  unless  flies  had  previous  access  to  the  meat, 
innumerable  microscopic  forms  of  life  did  appear  in  it,  in  spite 
of  all  efforts  to  exclude  them,  even  when  the  meat  was  care- 
fully and  hermetically  sealed.  Some  of  the  early  experimenters 
naturally  concluded  that  these  microscopic  forms  of  life  ap- 
peared spontaneously,  while  others  insisted  that  these  little 
organisms  had  found  entrance  into  the  sealed  vessels  from  the 
outside,  in  spite  of  all  precautions  taken  to  keep  them  out. 
Great  ingenuity  was  shown  in  devising  experiments  for  settling 
this  question.  The  results  obtained  by  different  experimenters 
were  in  great  conflict  for  over  two  centuries,  and  apparently 
equally  good  evidence 
was  found  both  for  and 
against  the  belief  in 
spontaneous  genera- 
tion. 

Needham  and  Spal- 
lanzani. — The  general 
method  used  by  the 
experimenters  was  to 
place  meat,  hay  infu- 
sions, cheese,  etc.,  in 
flasks,  and  then  by 
boiling  to  attempt  to 
kill  all  life  in  the  ma- 
terial, and  later,  by 
sealing  hermetically, 
to  guard  against  the  entrance  of  any  form  of  microscopic  life  from 
without.  But  even  under  these  conditions  it  was  frequently  found 
that  microscopic  life  made  its  appearance  in  the  sealed  vessels ; 
Fig.  2.  It  proved  very  difficult  to  be  sure  that  nothing  was  left 


FIG.  2. —  APPARATUS  USED  BY  SCHWANN  IN 
EXPERIMENTING  ON  SPONTANEOUS  GENERA- 
TION 

Steam  produced  by  boiling  passed  out  through  the 
tube,  but  upon  cooling  was  drawn  in  again  through  the 
heated  coil,  which  sterilized  it. 


14  BIOLOGY 

alive  in  the  material  after  boiling, —  i.e.,  that  it  was  sterile, 
—  and  to  be  sure  that  the  sealing  was  effectual.  Two  names 
especially  connected  with  this  dispute  were  Needham,  in  1749, 
and  Spallanzani,  in  1777.  Needham  believed  firmly  in  spon- 
taneous generation,  while  Spallanzani  insisted  that  the  micro- 
scopic organisms  that  appeared  in  these  experiments  were 
either  there  originally  and  not  killed  by  the  boiling  to  which 
the  material  had  been  subjected,  or  had  found  their  way  into 
the  solutions  through  microscopic  cracks  left  by  the  imperfect 
sealing. 

Pasteur  and  Appert. — In  the  middle  of  the  last  century  the 
French  scientist,  Pasteur,  carried  out  a  series  of  experiments 
and  attained  results  which  conclusively  disproved  the  theory 
of  spontaneous  generation.  But  the  long  debated  question 
would  not  be  settled  even  then.  It  is  a  curiously  interesting 
fact  that,  while  scientists  were  disputing  over  this  matter,  the 
question  had,  for  practical  purposes,  actually  been  settled  by 
Appert,  who  in  1831  had  discovered  the  method  of  preserving 
animal  and  vegetable  foods  by  the  means  of  heat  and  sealing, — 
the  method  used  by  the  canning  industries  of  the  present  day. 
But  the  significance  of  this  practical  discovery  was  not  appre- 
ciated, and  the  dispute  continued  even  after  Pasteur's  work, 
the  advocates  of  spontaneous  generation  continuing  as  insistent 
in  their  claims  as  ever.  The  settlement  of  the  question 
was  not  reached  until  the  English  physicist,  Tyndall,  devised 
a  new  and  ingenious  method  of  experimenting  which  so  satis- 
factorily guarded  all  sources  of  error  that  criticism  was  silenced. 
Indeed,  so  convincing  were  his  experiments  that  his  conclusions 
have  practically  never  been  questioned. 

Tyndall's  Experiments. — Briefly,  Tyndall's  method  of  ex- 
perimenting was  as  follows:  An  airtight  box  was  constructed, 
rectangular  in  shape  and  provided  at  either  end  and  in  front 
with  glass  windows.  Into  the  top  of  this  box  passed  small 
glass  tubes  which  had  been  thrown  into  several  curves,  through 
which  the  air  was  allowed  to  enter  freely;  Fig.  3  a. 


THE  SCOPE  OF  BIOLOGY 


15 


Recognizing  that  the  great  source  of  error  in  these  experi- 
ments was  due  to  the  germ-bearing  dust  of  the  air,  Tyndall 
attempted  to  free  the 
air  from  dust  by  coating 
the  inside  of  the  curved 
tubes  with  glycerine,  to 
entangle  the  dust  particles 
of  the  air  as  it  passed  up 
and  down  the  series  of 
curves  into  the  box.  This 
method  proved  to  be  suc- 
cessful, for  experiment  and 
microscopic  study  showed 
that  no  dust  passed  be- 
yond the  second  curve  of 
the  tubes.  The  interior  of 
the  box  was  also  coated 
with  glycerine,  so  that  the 
dust  particles  which  either 
settled  to  the  bottom  or 


FIG.  3. —  APPARATUS  USED  BY  TYNDALL 

For  description  see  text. 


floated  against  the  side  or  top  of  the  box  would  be  caught  in  the 
glycerine.  In  this  way  Tyndall  argued  that  he  could  obtain, 
in  time,  air  perfectly  free  from  germ-bearing  particles. 

He  did  not  wish  to  begin  an  experiment  until  the  air  in  the 
box  was  absolutely  free  from  dust,  and  in  order  to  determine 
this  point  the  two  glass  windows  at  the  end  of  the  box  were 
used.  A  ray  of  light  was  thrown  through  the  box,  in  at  one 
window,  and  out  through  the  other.  Thus,  any  dust  particles 
that  remained  floating  in  the  air  of  the  box  would  be  illumined 
and  made  clearly  visible  through  the  window  in  front.  At 
first  there  were  many  dust  particles  to  be  seen  floating  in  the 
air;  but  after  the  box  had  remained  quiet  for  several  days, 
the  ray  of  light  was  invisible  as  it  passed  through  the  box, 
proving  that  no  floating  dust  particles  were  present  to  be 
illumined.  When  this  condition  was  reached  Tyndall  assumed 


16  BIOLOGY 

that  the  air  was  sterile,  that  is,  pure,  so  far  as  any  floating 
particles  were  concerned,  and  that  his  box  was  ready  for 
experiment. 

At  the  bottom  of  the  box  were  a  series  of  tubes  whose  mouths 
opened  into  the  box  but  whose  lower  ends  projected  below; 
Fig.  3  b.  By  means  of  the  long  tube,  c,  which  could  be 
moved  to  and  fro  (since  it  passed  through  a  rubber  diaphragm, 
d),  all  the  test  tubes  could  be  filled  successively  with  any  of 
the  solutions  with  which  he  wished  to  experiment.  In  these 
tests,  Tyndall  used  various  materials:  old  meat,  old  cheese, 
hay  infusion,  etc.,  besides  many  other  substances  that  pre- 
vious experimenters  had  used  in  their  attempt  to  settle  the 
question.  After  filling  the  tubes  with  these  various  materials, 
they  were  heated  to  a  temperature  sufficiently  high  to  destroy 
all  life  that  they  might  have  contained  in  the  beginning.  This 
was  easily  done,  since  the  lower  end  of  the  tubes  projected 
below  the  level  of  the  box  and  could  be  very  easily  put  into 
a  bath  of  oil  or  brine,  and  heated  to  any  desired  temperature. 
Any  steam  or  vapor  that  might  arise  from  the  open  end  of 
the  test  tube  would  pass  into  the  box  and  readily  find  exit 
through  the  glass  tube  at  the  top.  Upon  cooling,  a  fresh 
supply  of  air  would  be  drawn  back  into  the  box  through  the 
curved  tube  a,  but,  as  already  indicated,  no  dust  particles 
would  find  entrance.  Having  thus,  by  heat,  killed  any  living 
organisms  that  might  be  in  the  solutions  to  be  tested,  he 
again  set  the  boxes  aside  and  watched  day  by  day  to  see 
what  would  happen.  Since  everything  was  clearly  visible  to 
the  eye,  it  was  possible  to  determine  very  quickly  and  surely 
whether  any  living  organisms  developed  in  the  test  tubes. 

Tyndall's  care  in  his  experiments  was  so  great  that  they  were 
quite  beyond  criticism.  His  experiments  showed  the  cause  of 
previous  errors  and  explained  why  there  had  been  such  con- 
flict in  the  earlier  experiments.  He  demonstrated  among  other 
things  that  some  forms  of  life,  called  spores,  might  remain 
alive  in  boiling  water  for  some  time.  This  conclusion  had  been 


THE  SCOPE  OF  BIOLOGY  17 

previously  reached  by  others;  but  Tyndall  proved  definitely 
that  while  a  temperature  below  boiling  is  sufficient  to  kill 
active  germs,  the  spores  stand  a  temperature  of  boiling  for  a 
long  time,  and  hence  boiling  does  not  sterilize  liquids.  Since 
previous  experimenters  had  assumed  that  all  life  was  destroyed 
by  boiling,  they  had  been  contented  with  the  simple  boiling 
of  the  liquid  to  eliminate  any  organisms  that  might  have  been 
there  originally.  If,  therefore,  any  of  these  resisting  spores 
chanced  to  be  in  their  solutions,  they  would  subsequently 
develop;  and  from  this  fact  the  experimenter  might  reach  the 
erroneous  conclusion  that  the  living  organisms  coming  from 
these  spores  developed  spontaneously.  Tyndall  carefully 
eliminated  all  of  these  errors  and  established  the  following 
important  conclusions.  No  evidence  for  spontaneous  genera- 
tion exists  and  the  success  of  an  experimenter  in  obtaining 
any  evidence  of  spontaneous  generation  is  in  inverse  propor- 
tion to  the  care  with  which  he  performs  his  experiments. 

This  statement  has  stood  almost  unquestioned  by  biol- 
ogists since  it  was  first  promulgated  in  1875;  and  during 
the  last  thirty  years  the  work  of  thousands  of  experimenters 
in  the  science  of  bacteriology  has  only  confirmed  the  accuracy 
of  Tyndall's  conclusion. 

We  must  accept  the  fact  that  whenever  any  living  animal 
or  plant,  no  matter  how  small,  makes  its  appearance  in  a 
solution,  originally  there  was  present  in  this  solution  a  living 
germ  which  started  the  development  of  the  organism  by  the 
process  of  ordinary  reproduction  and  growth.  At  the  present 
time,  therefore,  there  is  no  shred  of  evidence  that,  under  any 
conditions  which  we  can  produce,  life  can  arise  spontaneously. 

The  Primal  Origin  of  Life. — The  conclusion  that  spontaneous 
generation  does  not  occur  to-day,  leaves  unanswered  the  ques- 
tion of  the  primal  origin  of  life.  It  has  been  a  disappointment 
to  biologists  to  be  obliged  to  admit  that  they  can  find  no  evi- 
dence for  the  theory  of  spontaneous  development,  since  at  some 
period  in  the  history  of  the  world,  life  must  have  made  its 


18  BIOLOGY 

appearance  for  the  first  time.  In  an  early  period  of  the  world's 
history,  the  earth  was  a  hot,  molten  mass,  and  under  these 
conditions  no  living  matter  could  exist.  It  follows,  then,  that 
life  must  have  made  its  appearance  after  the  earth  had  suffi- 
ciently cooled.  Biology,  in  endeavoring  to  explain  life  by 
natural  forces,  has  been  eager  to  believe  that  in  these  earlier 
conditions  of  the  world  the  first  living  thing  may  have  ap- 
peared as  the  result  of  natural  law.  The  fact  that  biologists 
have  almost  universally  accepted  Tyndall's  conclusion  that 
no  evidence  for  spontaneous  generation  exists,  is  thus  a  testi- 
mony, both  to  the  truth  of  this  conclusion  and  to  the  honesty 
of  the  scientists  who  have  accepted  it.  They  would  have  much 
preferred  a  conclusion  of  the  opposite  kind.  The  majority  of 
biologists,  however,  believe  it  to  be  logically  necessary  to  as- 
sume that  at  some  time  in  prehistoric  ages,  the  first  living 
thing  appeared  from  a  source  which  was  not  living.  While 
accepting  the  fact  that  abiogenesis  does  not  occur  at  the  present 
day  or  under  present  conditions,  biologists  still  claim  that  we 
have  no  means  of  knowing  what  may  have  occurred  under 
different  conditions  in  earlier  eras  of  the  world's  history.  Thus, 
the  problem  of  the  primal  origin  of  living  matter  still  remains 
unsolved. 

THE  BIOLOGICAL  SCIENCES 

Since  the  science  of  biology  deals  with  all  living  matter,  it 
might  broadly  be  defined  as  the  study  of  life  in  all  its  phases. 
With  this  comprehensive  definition,  biology  can  be  made  to 
cover  nearly  the  whole  field  of  human  knowledge — most  sciences 
and  even  philosophy  —  including  not  only  everything  which 
relates  to  the  life  of  man,  but  all  that  concerns  the  life  of  the 
animal  and  plant  world  as  well.  But  for  practical  convenience 
in  study,  the  field  of  biology  is  usually  restricted  to  a  group  of 
definitely  related  sciences, — the  so-called  biological  sciences, — 
and  although  within  this  group  there  are  to  be  found  many 
ill-defined  boundary  lines,  and  much  overlapping  and  division 


THE  SCOPE  OF  BIOLOGY  19 

into  sub-groups,  the  sciences  which  compose  it  may  be  enumer- 
ated as  follows:  morphology,  with  its  sub-groups:  anatomy, 
histology,  taxonomy,  distribution,  structural  embryology;  and 
physiology,  with  its  sub-groups:  physiology  proper,  functional 
embryology,  psychology,  ecology,  and  sociology.  (See  reference 
chart,  p.  21.) 

MORPHOLOGY 

Morphology  (Gr.  morphe  =  form  +  -logia  =  discourse)  is  that 
branch  of  biology  which  deals  with  the  structure  and  form  of 
animals  and  plants.  It  may  be  divided  into  five  sub-heads: 

1.  Anatomy  (Gr.  ana  =  up  +  temnein  =  to  cut)  is  the  study 
of  all  of  the  grosser  structure  of  animals  and  plants,  that  can  be 
seen  and  dissected  without  the  aid  of  the  microscope. 

2.  Histology  (Gr.  histos  =  a  web  +  -logia)  is  the  study  of  the 
minute  structure  of  animals  and  plants  which  is  disclosed  only 
by  the  aid  of  the  microscope.    It  is  sometimes  called  microscopic 
anatomy  and  deals  chiefly  with  cell  structure. 

3.  Taxonomy  (Gr.  taxis  =  arrangement  +  nomos  =  law)  is 
the  study  of  the  relations  of  the  organisms  to  each  other  and 
includes  the  classification  of  species. 

4.  Distribution  is  the  study  of  the  geographical  distribution 
of  organisms  at  the  present  time,  and  also  their  distribution  in 
the  past  as  disclosed  by  geology;  to  the  latter  study  is  given  the 
name  paleontology. 

5.  Embryology  (Gr.  embryon  =  an  embryo  +  -logia)  is  the 
study  of  the  development  of  the  organism  from  the  egg  to 
the  adult  life.     It  is  also  called  ontogeny  (Gr.  on  (ont)  =  be- 
ing +  -geneia  =  producing)  in  distinction  from  phytogeny  (Gr. 
phylon  =  race  -\--geneia  =  producing),  the  development  of  the 
race. 

PHYSIOLOGY 

Physiology  (Gr.  physis  =  nature  +  -logia)  is  the  study  of  the 
activities  or  functions  of  organisms.  Its  scope  may  be  best 
understood  by  its  division  into  sub-heads: 


20  BIOLOGY 

1.  General  physiology.    Physiology  deals  primarily  with  the 
functions  of  the  different  organs.     Correctly  used,  it  should 
include  the  functions  of  all  animals  and  plants.    Since,  however, 
human  physiology  has  been  so  much  more  studied  than  that  of 
other  animals,  the  term  physiology  usually  refers  to  mankind. 
When  the  study  extends  to  other  animals  or  to  plants,  it  is  des- 
ignated respectively  as  animal  physiology  and  plant  physiology. 

2.  When  embryology  concerns  itself  with  the  activities  of 
the  embryo,  it  then  belongs  to  the  domain  of  physiology. 

3.  Psychology  (Gr.  psyche  =  soul  +  -logia)  is  the  study  of 
the  functions  of  the  brain.    It  includes  not  only  the  study  of  the 
human  brain  but  the  brain  activities  of  other  animals  as  well, 
under  the  term  comparative  psychology. 

4.  Ecology  (Gr.  oikos  —  house  -f  -logia)  is  the  study  of  the 
relations  of  organisms  to  their  environment.    This  includes  their 
relations  to  inanimate  nature  as  well  as  to  animate.     The  term 
ecology  is  now  more  widely  applied  in  relation  to  plants  than  to 
animals.    Ecology  includes  sociology  (Lat.  socim  =  a  compan- 
ion +  Gr.  -logia),  which  is  the  study  of  the  interrelations  of  ani- 
mals of  the  same  species.    This,  however,  is  chiefly  confined  to 
the  human  race,  the  term  sociology  usually  referring  to  mankind. 
There  are,  however,  some  animals  like  ants,  bees,  etc.,  that  have 
social  relations,  and  the  term  sociology  might  be  extended  to 
them. 

ZOOLOGY  AND  BOTANY 

The  general  term  zoology  includes  any  of  the  biological  sci- 
ences when  studied  in  their  relation  to  animals,  and  the  general 
term  botany,  when  they  are  studied  in  their  relation  to  plants. 


THE  SCOPE  OF  BIOLOGY 

BIOLOGY 

The  science  of  living  things 


21 


MORPHOLOGY 

The  science  of  form 


PHYSIOLOGY 

The  science  of  function 


Anatomy 

General  Physiology 

The  study   of 

The  study  of  func- 

gross structure 

tions  of  organs 

Histology 
The  study  of  mi- 
nute structure 

includes 

Embryology 
(functional) 

The  study  of  the 
activities  of  the 

Taxonomy 

| 

embryo 

The  classification 

N       § 

Ecology 

N 

of  species 

1 

The  study  of  the 

I1 

Distribution 

i?5. 

relation  of  organ- 
isms to  their  en- 

1 

The  geographical 

g1 

vironment 

and  chronological 

1* 

1" 

relation  of  organ- 
isms 

Sociology 
The  study  of 

interrelations 

Embryology 

of    animals 

(structural) 

of   the   same 

The  study  of  de- 

species 

velopment    from 
the  germ 

Psychology 
The   study  of 

brain  functions 

22  BIOLOGY 

LABORATORY    WORK  WITH    ORGANIC   COMPOUNDS 

PROTEIDS 

Albumen. — Separate  a  little  of  the  white  from  the  yolk  of  an  egg  and 
dilute  with  three  times  its  quantity  of  water.  With  this  solution  make 
the  following  tests: 

1.  Place  a  little  of  the  albumen  solution  in  a  test  tube  and  boil,  noting 
that  a  precipitate  appears;  that  is,  the  albumen  coagulates.     Repeat  this 
test,  heating  the  albumen  in  a  test  tube  in  a  water  bath,  determining, 
by  a  thermometer  placed  in  the  test  tube,  at  what  temperature  the  coagu- 
lation occurs. 

2.  Add  a  little  strong  HNO3  to  some  of  the  albumen  in  a  test  tube. 
A  precipitate  appears.    Boil,  and  the  precipitate  will  turn  yellow.    Allow 
it  to  cool  and  add  enough  ammonia  to  neutralize  the  acid  and  it  will  turn 
a  deep  orange.    This  is  known  as  the  xanthoproteic  test  for  proteids. 

3.  To  a  weak  solution  of  albumen  add  a  few  drops  of  NaOH  and  a 
few  drops  of  a  1%  solution  of  CuSO4;  heat  gently  and  the  solution  will 
turn  blue  if  ordinary  proteids  are  present,  but  if  peptones  are  present  it 
will  show  a  reddish  color. 

Gluten. — Place  some  flour  in  a  large  piece  of  cheesecloth,  and  gather- 
ing up  the  edges  of  the  cloth,  wash  thoroughly  in  a  pail  of  water.  Much 
of  the  bulk  of  the  flour  will  wash  away,  but  the  gluten  will  finally  be  left 
in  the  cloth,  as  a  sticky  mass  that  will  not  wash  out. 

Remove  a  little  of  the  wash  water  from  the  pail  in  a  test  tube  and  add 
a  few  drops  of  iodine  to  it.  If  it  turns  blue  it  will  indicate  the  presence 
of  starch. 

Casein.— Add  a  little  2%  HCL  to  a  few  c.  c.  of  milk.  A  curd  will  form 
whioh  can  be  separated  from  the  liquid  by  allowing  it  to  drain  through 
cheesecloth.  The  curd  is  the  proteid,  casein. 

Myosin. — Soak  some  chopped  beef  in  cold  water  for  half  an  hour; 
stir  and  filter  through  cheesecloth.  Boil  the  filtrate,  and  a  mass  of  myosin 
will  appear,  which  was  dissolved  in  the  cold  water  but  is  coagulated  by 
heat. 

Fibrin. — This  is  a  proteid  formed  from  blood.  It  may  be  obtained  by 
collecting  freshly  drawn  blood  and  stirring  it  immediately  with  a  piece 
of  wire  gauze  for  about  ten  minutes.  A  mass  of  fibrin  will  collect  on  the 
wire,  and  it  will  be  found  that  the  blood  will  not  subsequently  clot,  the 
removal  of  the  fibrin  preventing  it. 

CARBOHYDRATES 

Starch. — Rub  up  a  little  starch  (potato  starch  is  best)  in  an  evaporating 
dish  with  a  considerable  quantity  of  water.  Place  a  few  drops  in  a  teat 
tube  and  add  a  little  iodine  solution.  The  starch  will  turn  blue. 


THE  SCOPE  OF  BIOLOGY  23 

Examine  a  little  of  the  starch  water  under  a  microscope.  Sketch  some 
of  the  starch  grains.  Make  a  thin  section  of  a  bit  of  potato  with  a  razor 
and  examine  under  a  microscope,  noting  the  starch  grains.  Add  a  little 
iodine  solution  and  again  examine  with  a  microscope. 

Boil  the  starch  water  over  a  flame.  As  it  comes  near  to  the  boiling 
point  the  mass  will  become  thick  and  pasty  (starch  paste),  due  to  the 
bursting  of  the  starch  grains  by  heat.  Place  a  little  under  the  microscope 
and  look  for  grains.  Add  to  the  paste  a  little  iodine  and  it  will  turn  a 
brilliant  blue. 

Test  for  Sugar. — Put  a  little  glucose  or  dextrose  in  a  test  tube  con- 
taining a  considerable  quantity  of  water.  Add  to  this  a  few  drops  of 
weak  H2SO4  and  a  few  drops  of  NaOH;  boil.  The  presence  of  sugar  is 
determined  by  the  appearance  of  a  brownish  red  precipitate,  which  goes 
through  a  series  of  color  changes,  but  finally  remains  as  a  brownish  red 
sediment  at  the  bottom  of  the  tube. 

FATS 

One  of  the  simplest  tests  for  fat  is  to  place  the  material  in  which  the 
fat  is  supposed  to  be  upon  a  sheet  of  common  paper.  The  paper  will  be 
rendered  transparent  by  absorbing  the  oil  of  the  fat-containing  tissue. 

Fat  Emulsion. — Fat  has  the  property  of  being  readily  divided  into 
minute  particles  which,  when  mixed  with  water,  float  in  the  liquid,  form- 
ing what  is  known  as  an  emulsion.  Place  a  few  drops  of  olive  oil  in  a 
test  tube  half  full  of  water.  The  oil  will  rise  to  the  top  of  the  water  and 
appear  as  a  clear  yellowish  layer.  Put  a  finger  over  the  mouth  of  the 
test  tube  and  shake  vigorously.  The  whole  contents  of  the  tube  will 
turn  a  milky  white,  and  upon  being  allowed  to  stand  the  milkiness  will 
remain  for  a  long  time.  Eventually,  however,  the  fat  again  separates 
from  the  water.  This  milky  appearance  is  produced  by  the  fact  that  the 
fat  has  been  divided  into  minute  particles  that  float  through  the  water 
and  refract  the  light  in  such  a  way  as  to  give  a  white  color.  This  is  called 
an  emulsion. 

Examine,  under  a  microscope  with  a  high  power,  a  drop  of  milk,  noting 
that  it  is  an  emulsion. 


BOOKS  FOR  REFERENCE 
Recent  Works  in  General  Physiology  and  Psychology 

CONN,  Advanced  Physiology,  Silver,  Burdett  &  Co.,  New  York. 
DRIESCH,    The   Science   and   Philosophy   of   the   Organism,  A.  &  G 
Black,  London. 


24  BIOLOGY 

FOSTER,  Text-book  of  Physiology,  Macmillan  Co.,  New  York 

JAMES,  Psychology,  Vols.  I  and  II,  Henry  Holt  &  Co.,  New  York. 

JUDD,  Psychology,  General  Introduction,  Chas.  Scribner's  Sons,  New 
York. 

McCABE,  The  Evolution  of  Mind,  A.  &  C.  Black,  London. 

MEYERS,  Text-book  of  Experimental  Psychology,  Longmanns,  Green  & 
Co.,  London. 

PILLSBURY,  The  Essentials  of  Psychology,  Macmillan  Co.,  New  York. 

TIEGERSTADT,  Text-book  of  Physiology,  Appletons,  New  York. 

TITCHNER,  Text-book  of  Psychology,  Experimental  Psychology,  Vols.  I 
and  II,  Macmillan  Co.,  New  York. 

VERWORN,  General  Physiology,  Macmillan  Co.,  New  York. 

WUNDT,  Physiologische  Psychologic,  Vols.  I-III,  Outlines  of  Psy- 
chology (translation),  Engelmann,  Leipzig. 

Important  Works  Bearing  upon  Abiogenesis 

SWAMMERDAM  (1637-1680),  Bibel  der  Natur,  1652. 

REDI,  Esperinze  interno  alia  generazione  degl'  insetti,  1688. 

LEEUWENHOEK  (1632-1723),  Arcana  naturae,  1695. 

NEEDHAM,  Observations  upon  the  Generation,  Composition,  and  De- 
composition of  Animals  and  Vegetable  Substances,  London,  1749. 

BONNET  (1720-1793),  Considerations  sur  les  corpes  organises,  1762. 

SPALLANZANI  (1729-1799),  Physicalische  und  mathematische  Abhand- 
lungen,  1769. 

SCHULTZE  (1825-1874),  Vorlaufige  Mittheilung  der  Resultate  einer 
experimentellen  Beobachtung  iiber  generatio  sequivoca,  Gilbert's  Annalen, 
1836. 

SCHWANN  (1810-1882),  Vorlaufige  Mittheilung,  betreffend  Versuch  iiber 
die  Weingahrung  und  Faulnis,  Gilbert's  Annalen,  1837. 

PASTEUR  (1822-1895),  Comptes  Rendus,  Vol.  50,  1861. 

BASTIAN,  The  Beginnings  of  Life,  1872. 

TYNDALL  (1820-1893),  Philosophical  Transactions,  1876,  1877;  Floating 
Matter  in  the  Air,  1882. 

Useful  Laboratory  Manuals 

ANDREWS,  Practical  Course  in  Botany,  American  Book  Company, 
New  York. 


THE  SCOPE  OF  BIOLOGY  25 

CALKINS,  Protozoology,  Lea  &  Febiger,  New  York. 

COLTON,  Zoology,  Descriptive  and  Practical,  D.  C.  Heath  &  Co., 
Boston. 

DAVIDSON,  Practical  Zoology,  American  Book  Company,  New  York. 

HAWK,  Physiological  Chemistry,  P.  Blakiston's  Son  &  Co.,  Philadelphia. 

LONG,  Text-book  of  Physiological  Chemistry,  P.  Blakiston's  Son  & 
Co.,  Philadelphia. 

MELL,  Practical  Laboratory  Methods,  Macmillan  Co.,  New  York. 

PARKER  and  PARKER,  Practical  Zoology,  Macmillan  Co.,  New  York. 

PAYNE,  Manual  of  Experimental  Botany,  American  Book  Company, 
New  York. 

PRATT,  Invertebrate  Zoology,  Ginn  &  Co.,  Boston,  Mass.. 

RICHARDS  and  WOODMAN,  Air,  Water,  and  Food,  Wiley  &  Son,  New 
York. 

ROCKWOOD,  Laboratory  Manual  of  Physiological  Chemistry,  F.  A.Davis, 
Philadelphia. 

SHARPE,  Laboratory  Manual  in  Biology,  American  Book  Company, 
New  York. 


CHAPTER  II 

CELLS  AND  THE  CELL  THEORY 
ORGANISMS 

ONE  characteristic  feature  of  living  matter  is  that  it  is  not 
indefinitely  distributed  around  the  world,  but  is  always  asso- 
ciated in  distinct  units  or  individuals.  In  other  words,  there 
is  no  life  apart  from  individuals.  These  units  always  contain 
different  parts,  each  with  a  distinct  function.  This  is  very 
evident  among  well-known  animals  and  plants.  The  human 
body  possesses  a  heart,  a  stomach,  a  brain;  and  a  tree  has 
roots,  leaves,  flowers,  etc.  These  different  parts  are  called 
organs ;  and  because  it  possesses  organs,  a  living  being  is  called 
an  organism.  While  it  is  true  that  practically  all  living  things 
do  have  organs,  some  of  the  lowest  are  so  small  that  no  organs 
have  yet  been  found  in  them,  as  for  example,  bacteria;  see 
Fig.  7.  It  is  probable,  however,  that  these  do  have  organs 
if  we  were  only  able  to  see  them;  at  all  events,  the  term  organ- 
ism is  extended  to  all  living  things  whether  they  possess  evi- 
dent organs  or  not. 

From  the  word  organisms  is  coined  the  adjective  organic,  that 
is,  pertaining  to  organisms.  Organic  substances  have  been  pro- 
duced by  living  beings,  while  inorganic  substances  have  no  con- 
nection with  living  things.  Bone,  muscle,  wood,  sugar,  coal,  etc., 
are  organic;  while  stones,  water,  and  air  are  inorganic.  Nearly 
all  organic  substances  contain  carbon  and  are  capable  of  being 
burned,  while  inorganic  substances  usually  contain  no  carbon. 

THE  CELL  AS  THE  UNIT   OF   ORGANIC   STRUCTURE 

The  slightest  familiarity  with  the  larger  well-known  animals 
and  plants  shows  not  only  that  they  are  made  up  of  different 
organs,  each  with  its  definite  duty  to  perform,  but  also  that  these 
organs  are  composed  of  different  parts,  each  having  its  specific 

26 


CELLS  AND  THE  CELL  THEORY 


27 


function.  The  stomach  has  its  muscles  and  its  secreting  glands; 
the  foot  has  its  muscles,  bones,  tendons,  ligaments,  nerves,  etc. 
The  different  kinds  of  substance  which  form  the  organs  are 
known  as  tissues,  and  usually  each  tissue  contains  only  one  kind 
of  material  and  has  but  one  kind  of  duty  to  perform.  For 
example:  muscles,  bones,  glands,  nerves, 
and  tendons,  each  represent  a  distinct 
tissue;  each  has  its  special  function  in 
the  organ,  and  each  is  different  from  the 
other.  Muscles  have  the  power  of  con- 
traction, bones  are  for  support,  etc. 

By  studying  these  different  tissues  un- 
der the  microscope  we  shall   find   that 
they,  too,  are  made  up  of  minute  parts, 
called  cells,  and  that  in  most  instances    iNG  CARTILAGE  TISSUE 
each  cell  is  essentially  like  all  the  other 
cells  of  the  same  tissue.     This  may  be  shown  by  examining 
Figures  4  to  6,  in  which  several  kinds  of  tissue  appear,  each  made 


FIG.  4. —  CELLS  FORM- 


FIG.  5. —  CELLS  FORMING  BONY  TISSUE 

up  of  a  large  number  of  independent,  similar  cells.    These  cells 
represent  the  ultimate  units  to  which  the  analysis  of  the  struc- 


28 


BIOLOGY 


ture  of  living  things  has  been  carried  at  present;  for  while  each 
cell  is  made  up  of  parts,  life  as  a  whole  seems  to  be  found  only 
where  we  have  the  whole  structure  of  the  cell  developed.  In 
other  words,  the  cell  is  the  simplest  form  in 
which  life  occurs,  and  is,  in  this  sense,  the  ulti- 
mate unit  of  living  structure.  While  an  organ 
may  contain  many  different  kinds  of  cells,  each 
tissue  is,  as  a  rule,  made  of  but  one  kind  of  cell. 
The  cells  of  the  bone,  for  example,  are  all  essen- 
tially alike,  and  so,  too,  are  the  cells  of  muscles 
and  glands.  The  different  cells  in  the  same  tissue 
may  differ  in  shape  and  size;  but  these  differences 
are  only  superficial ;  fundamentally  the  cells  form- 
ing a  single  tissue  are  alike.  Therefore,  if  we  de- 
fine a  cell  as  the  ultimate  unit  in  the  analysis 
of  living  structure,  we  may  define  a  tissue  as  an 
aggregate  of  similar  cells,  all  having  similar  func- 
tions; see  Figs.  4,  5,  and  6. 

While  the  form,  structure,  and  size  of  cells 
present  an  almost  endless  variety,  in  both  the 
animal  and  plant  worlds,  nevertheless,  all  cells 
have  in  common  certain  general  parts.    Thus  we 
may  speak  of  the  structure  of  a  cell  in  general,  recognizing  that 
all  living  cells  of  both  animals  and  plants,  in  spite  of  their 
differences,  conform  essentially  to  the  type  of  an  ideal  cell. 


FIG.  6.— CELLS 

FORM  ING 
MUSCLE  TIS- 
SUE FROM 
THE  INTES- 
TINE WALL 


CELL  STRUCTURE 

The  description  given  below  is  not  that  of  any  particular  cell, 
but  rather  that  of  a  typical  or  ideal  cell.  Though  a  cell  exactly 
like  that  described  will  not  be  found,  it  resembles  closely  the 
cell  which  forms  the  egg  of  certain  animals,  and  in  essential 
structure  is  like  all  cells  found  in  animals  and  plants. 

Structure. — The  cell  consists  of  four  primary  parts,  some  of 
which  may  be  absent: — 


CELLS  AND  THE  CELL  THEORY 


29 


1.  The  protoplasm,  or  cell  substance,  a  liquid  making  up  the 
bulk  of  the  cell. 

2.  The  nucleus,  a  rounded  body  within  the  cell  substance. 

3.  The  centrosome,  a  small  body  near  the  nucleus. 

4.  The    cell  wall,  an  outer  covering  which  holds  the  cell 
substance.       (The   cell  wall   and   centrosome  are  sometimes 
absent.) 

Size. — There  is  much  variation  in  the  size  of  cells.  Some  of 
them  are  extremely  minute. 
Bacteria,  which  are  sometimes 
not  more  than  1/50,000  of  an 
inch  in  diameter,  are  probably 
cells  (Fig.  7),  although  we  do 
not  yet  know  positively  that 
they  contain  a  nucleus  and  cen- 
trosome. At  all  events  the 
yeast,  which  is  only  a  little 
larger  than  a  bacterium  (about 
1/3000  of  an  inch),  is  a  typical 
cell,  possessing  a  nucleus,  cell 
wall,  and  cell  substance;  see 
Fig.  32.  At  the  other  end  of 
the  scale  we  find  giant  cells, 


FIG.  7. —  BACTERIA  VERY  HIGHLY 
MAGNIFIED 

Showing  the  complex  internal  structure  with 
bodies  supposed  by  some  to  be  nuclei.  At  o 
one  cell  shows  what  resembles  karyokinetic 
division. 


FlG.    8. NlTELLA 

A,  about  natural  size,  showing  nodes 
and  internodes;  B,  one  of  the  inter- 
nodes  more  magnified.  The  part  en- 
closed by  brackets,  between  the  two 
rows  of  leaves,  is  a  single  cell. 


which  may  be  an  inch  in  length,  as  in  the  case  of  a  small 
plant  known  as  Nitella  (Fig.  8),  or  larger  still,  as  in  the  egg 


30 


BIOLOGY 


FlG.  9. — A  DIAGRAM  OP  AN  IDEAL  CELL 
I,  linin; 
m,  microsomata; 


as,  centrosphere ; 
ch,  chroma  tin; 
cr,  centrosome; 
cw,  cell  wall; 
cy>  cytoplasm; 
/,  fibers; 
ky,  karyoplasm; 


nm,  nuclear  membrane; 
n,  nucleus; 
no,  nucleolus; 
p,  plastids; 
v,  vacuole. 


stance)  has  been  given.     This  material 
within  the  cell  wall,  or  lie  in  the  form 


of  the  ostrich,  which  is 
really  a  single  cell.  As 
a  rule,  however,  cells 
are  microscopic  in  size. 
Shape. — A  cell  is  usu- 
ally more  or  less  spher- 
ical (Fig.  9),  although 
it  may  be  distorted  by 
pressure  or  irregular 
growth. 

CELL  SUBSTANCE  OR 
PROTOPLASM 

The  material  which 
composes  the  active 
part  of  the  cell  appears 
like  a  mass  of  more  or 
less  transparent  jelly  to 
which  the  name  pro- 
toplasm (Gr.  protos  = 
first  +  plasma  =  sub- 
may  fill  the  entire  space 
of  a  thin  layer  next  to 


FIG.  10. — CELL  OF  RPIROGYRA 

cl,  cell  chlorophyll;  cs,  cell  sap;  cw,  cell  wall;  n,  nucleus;  p,  protoplasm. 

the  cell  wall,   the  rest  of  the  space  being  filled  by  a  watery 
liquid;   Fig.  10. 


CELLS  AND  THE  CELL  THEORY 


31 


Structure. — When  protoplasm  is  examined  under  the  micro- 
scope it  is  not  found  to  be  a  homogeneous  jelly,  as  was  at  first 
thought,  but  to  have  an  intricate  structure  which  is  only  partly 
disclosed  by  the  microscope;  Fig.  11.  The  exact  structure  of 
this  cell  substance  has  not  been  fully  determined,  and  there 
are  at  least  three  different  theories  to  explain  its  microscopic 
appearance. 

The  Reticular  Theory. — One  school  of  scientists  describes  pro- 
toplasm as  an  extremely  minute  network  of  fibers  forming  a  sort 
of  sponge,  in  the  meshes  of  which 
there  is  found  a  moving  liquid; 
Fig.  II  A .  This  is  the  so-called 
reticular  or  fibrillar  theory  of  pro- 
toplasmic structure. 

The  Foam  Theory.  —  Another 
school  explains  the  appearance 
of  protoplasm  as  due  to  a  mass 
of  minute  bubbles,  like  soapsuds 
on  a  small  scale;  and  insists  that 
what  appear  to  be  fibers  are  only 
the  delicate  lines  separating  the 

bubbles  from  each  other;  Fig.  11 B.    This  is  the  foam  theory  of 
protoplasmic  structure. 

The  Granular  Theory. — Still  a  third  theory  suggests  that  the 
protoplasm  consists  of  an  indefinite  number  of  minute,  living, 
moving  granules,  arranged  in  lines  resembling  fibers  or  in  various 
other  figures.  This  is  the  granular  theory  of  protoplasmic 
structure. 

Between  these  theories  the  scientists  have  not  reached  any 
conclusion,  although  the  first  two  have  been  more  generally 
accepted  than  the  last.  It  is  quite  possible,  and  even  probable, 
that  all  of  the  theories  may  have  a  certain  amount  of  truth  in 
them,  and  that  protoplasm  does  not  in  all  cases  have  the  same 
structure.  It  is  certain,  however,  that  protoplasm  always  shows 
a  structure  and  is  not  a  homogeneous  body.  In  most  cases 


A  B 

FIG.  11. — DIAGRAMS  ILLUSTRAT- 
ING  THEORIES   OF   PROTOPLASM 
A,  the  Fibrillar;  B,  the  Foam. 
(Dahlgren  and  Kepner.) 


32  BIOLOGY 

two,  and  frequently  three,  distinct  substances  are  discernible 
in  it. 

1.  A  mesh  work  (reticulum)  resembling  fibers. 

2.  A  liquid  (cytoplasm)  occupying  the  meshes  of  the  net- 
work. 

3.  Minute    bodies  (microsomata)    (Gr.    micros  =  small  -f- 
soma  =  body)  scattered  along  the  branches  of  the  network, 
regularly  or  irregularly,  and  frequently  moving  to  and  fro  in  the 
cell. 

Activity  of  Protoplasm. — If  living  protoplasm  be  studied  under 
the  microscope,  it  will  frequently  show  a  type  of  motion  called 
streaming.  This  is  due  to  minute  granules  constantly  circu- 
lating in  a  more  or  less  definite  or  indefinite  fashion  within 
the  cell.  Whether  all  protoplasm  will  show  such  motion  we  do 
not  know,  but  apparently  whenever  this  substance  is  actually 
alive  this  motion  is  present.  Possibly  this  may  not  be  true  of 
protoplasm  that  is  known  as  dormant,  but  it  is  almost  certainly 
true  of  all  active  cells. 

THE  NUCLEUS 

Lying  within  the  cell  substance  there  is  a  smaller  body, 
usually  of  an  approximately  spherical  shape,  called  the  nucleus 
(Lat.  nucleus  =  nut);  Fig.  9n.  This  is  a  structure  of  extreme 
complexity.  It  is,  as  a  rule,  bounded  by  a  delicate  nuclear  mem- 
brane nm,  which  holds  the  contents  and  separates  them  from 
the  surrounding  cell  substance.  Within  this  membrane  may  be 
found  a  jelly-like  mass,  very  similar  to,  if  not  identical  with,  the 
cell  substance  outside,  and  also  included  under  the  term  pro- 
toplasm. To  distinguish  these  two  parts  of  the  protoplasm, 
that  inside  of  the  nucleus  is  called  karyoplasm  (Gr.  karyon  = 
nut  +  plasma  =  substance)  or  nucleoplasm,  while  that  outside 
is  called  cytoplasm  (Gr.  cytos  =  cell  +  plasma) ;  Fig.  9  ky  and  cy. 
In  addition  to  karyoplasm,  however,  there  are  other  distinct 
parts  in  the  nucleus.  Delicate  fibers  run  through  it  called  linin 
fibers  (Fig.  90,  and  a  small  rounded  body  known  as  the  nucle- 


CELLS  AND  THE  CELL  THEORY 


33 


olus  (Fig.  9  no)  is  usually  present  and  is  sometimes  very  promi- 
nent. The  significance  of  this  nucleolus  is  at  the  present  day 
unknown. 

The  most  remarkable  substance  in  the  nucleus  is  a  material 
known  as  chromatin  (Gr.  chroma  =  color);  Fig.  9  ch.  It  has 
received  the  name  chromatin  from  the  fact  that  it  has  a  special 
affinity  for  certain  staining  reagents,  the  chromatin  material  in 
the  nucleus  being  the  first  thing  to  absorb  the  color  and  become 
stained.  By  special  methods  the  chromatin  may  be  stained  and 
the  rest  of  the  nucleus  left  unstained.  The  latter  is  sometimes 
called  achromatin  (a  =  without  +  chroma  =  color).  By  this 
special  process  of  staining  it  is  possible  to  show  the  chromatin 
in  prepared  specimens,  although  in  the  living  cell  the  chromatin 


FIG.  12. —  NUCLEI,  SHOWING  THE  DIFFERENT  APPEARANCES  OP 
THE  CHROMATIN  (VARIOUS  AUTHORS) 

is  so  transparent  as  to  be  practically  invisible.  Chromatin 
occurs  in  a  great  variety  of  forms  in  different  nuclei.  Some  of 
these  are  shown  in  Figure  12.  It  is  sometimes  diffused  irregu- 


34  BIOLOGY 

larly  through  the  nucleus;  it  may  be  in  the  form  of  stars,  or  a 
long  coiled  thread,  or  it  may  appear  as  isolated  threads,  or  as 
threads  interlaced,  etc.  Whatever  its  form,  it  always  has  the 
power  of  absorbing  coloring  material  and  is  probably  always  of 
the  same  general,  chemical  composition.  The  nucleus  controls 
the  cell  activities,  and  the  chromatin  forms  the  most  impor- 
tant part  of  the  nucleus. 

THE  CENTROSOME 

Near  the  nucleus  in  many  cells  may  be  found  a  minute  body 
(Fig.  9cr)  known  as  the  centrosome  (Gr.  centron  =  center  + 
Gr.  soma  =  body),  which  is  usually  present  in  the  cells  of 
animals,  where  it  seems  to  have  an  important  function  in  con- 
trolling the  multiplication  of  the  cell.  The  centrosome  is 
usually  lacking  in  the  cells  of  the  higher  plants.  Frequently 
two  centrosomes  are  found  near  together,  and  sometimes  they 
are  surrounded  by  a  clear  area,  which  is  designated  as  the 
centrosphere.  At  one  time  the  centrosome  was  considered  of 
great  importance  in  the  life  of  the  cell,  from  its  prominent  role 
in  cell  division;  but  since  it  has  been  discovered  that  some  cells 
have  none,  while  others  have  several,  its  significance  as  an 
essential  element  in  cellular  structure  has  been  doubted. 

THE  CELL  WALL 

One  of  the  functions  of  the  cell  substance  in  many  cells  is  to 
secrete  around  the  cell  a  material  of  harder  consistency  than  the 
protoplasm,  the  cell  wall.  Some  cells  have  no  cell  wall;  for 
example,  the  animal  shown  in  Figure  13  is  a  cell  devoid  of  a  cell 
wall;  and  in  many  other  animal  cells  the  wall  is  either  very 
slight  or  entirely  lacking.  From  this,  it  is  evident  that  the  cell 
wall  cannot  be  regarded  as  an  essential  part  of  the  cell.  In 
nearly  all  vegetable  tissues,  the  living  protoplasm  secretes  a 
membrane  of  greater  or  less  consistency,  and  the  same  is  also 
true  of  many  animal  cells.  The  cell  wall  may  be  made  of  a 
variety  of  different  materials,  In  plants  it  is  sometimes  of  wood, 


CELLS  AND  THE  CELL  THEORY 


35 


or  of  a  material  allied  to  starch  and  known  as  cellulose.  Again 
it  may  be  composed  of  lime,  or  made  up  of  a  hornlike  substance, 
as  in  the  case  of  the 
cells  that  secrete  the 
finger  nails,  or  the  horns 
of  animals.  The  cell 
wall  is  not  alive,  being 
simply  a  secretion  of 
the  living  cytoplasm. 
The  cell  walls  may  be 
very  thin,  or  entirely 
absent  as  in  Figure  13. 
In  other  cases  they  may 
be  very  thick  and  form 
a  tissue  principally 
composed  of  cell  wall, 
with  only  scattered  bits 
of  b  /ing  protoplasm  in 
the  midst  of  a  great 

ma-ss  of  secreted  wall  substance.  This  is  especially  true  in  the 
case  of  the  cartilage,  as  shown  in  Figure  4.  The  shape  of  a 
cell  is  usually  determined  by  the  shape  of  its  cell  wall.  Figure  14 
shows  a  number  of  cells  and  gives  an  idea  of  the  various  shapes 
ihe  cell  wall  may  assume. 

\|  Since  the  cell  wall  is  lifeless  and  has  only  the  function  of  sup- 
port, the  cell  contents  alone  being  alive,  it  follows  that  any 
organism  may  contain  both  living  and  lifeless  material.  Among 
plants  the  lifeless  material  may  far  surpass  the  living  in  bulk. 
In  a  tree,  for  example,  most  of  the  trunk,  roots,  and  branches 
are  made  of  the  dead  walls  of  cells  which  were  formerly  filled 
with  living  protoplasm.  In  a  large  tree  only  a  thin  layer  of  cells 
directly  under  the  bark,  the  cells  found  in  the  leaves,  buds, 
and  some  cells  in  the  roots,  are  actually  alive.  In  animals  a 
much  larger  proportion  of  the  body  cells  are  alive,  the  bulk  of 
the  muscles  beirg  living  protoplasm ;  but  the  skin,  hair,  cartilage, 


FlG.    13. —  A   SINGLE-CELLED   ANIMAL 

ACTINOPHRYS 
A  cell  without  a  cell  wall. 


BIOLOGY 


F.IG.  14. — SHOWING  CELLS  OF  VARIOUS  SHAPES 

All  except  C  are  plant  cells;  C,  a  ciliated  cell  from  the  oesoph- 
agus of  an  animal.     (From  various  authors.) 


CELLS  AND  THE  CELL  THEORY          37 

and  bone  contain  in  a  marked  degree  lifeless  cell  walls  from 
which  the  living  matter  is  either  wholly  withdrawn,  as  in  the 
hair,  or  remains  only  in  a  relatively  small  amount,  as  in 
bone  and  cartilage. 

Other  Substances  in  a  Cell.— Cells  may  contain  other  bodies 
than  those  already  described,  which  cannot  be  regarded,  how- 
ever, as  essential  to  cell  life,  since  they  are  not  characteristic 
of  all  cellular  structure.  Some  of  these  are  called  plastids  (Fig. 
9  p) ,  and  seem  to  grow  and  divide  and  to  be  handed  on  from 
one  cell  generation  to  the  next.  Examples  of  such  plastids  are 
the  chlorophyll  bodies  in  plant  cells,  or  vacuoles  in  some  animals. 
Other  bodies  included  in  cells  are  purely  passive  bodies  which 
seem  to  be  functionless,  inert,  excreted  substances,  not  growing 
and  not  handed  down  from  generation  to  generation. 

CELL  FUNCTIONS 

The  cell  with  its  protoplasm  and  nucleus  contains  all  of  the 
parts  that  are  necessary  for  life,  and,  so  far  as  we  know,  nothing 
simpler  than  a  cell  is  capable  of  carrying  on  all  the  functions  of 
life.  If  this  be  true,  we  are  justified  in  saying  that  the  ideal  cell 
we  have  been  describing  is  the  simplest  bit  of  structural  machinery 
that  can  manifest  all  the  functions  of  life.  All  living  organisms, 
animals  and  plants  alike,  are  either  single  cells  (unicellular)  or 
complexes  of  cells  (multicellular),  and  the  life  of  the  organism 
as  a  whole  is  thus  the  combined  life  of  its  individual  cells. 

Definition  of  a  Cell. — To  sum  up,  then,  we  may  say:  A  cell  is 
a  combination  of  a  bit  of  protoplasm  (cytoplasm)  with  a  nucleus, 
and  it  is  the  simplest  structure  known  to  show  the  phenomena 
of  life. 

HISTORY  OF  THE  CELL  DOCTRINE 

The  development  of  the  cell  doctrine  may,  for  convenience,  be 
divided  into  three  periods : — 

1.    The  early  conception  of  the  cell,  1839  to  1861. 


38  BIOLOGY 

2.  The  discovery  of  protoplasm  and  the  development  of  the 
mechanical  theory  of  life,  1861  to  about  1885. 

3.  The  discoveries  of  the  functions  of  the  nucleus  and  its 
relations  to  reproduction  and  heredity,  from  about  1880  to  the 
present. 

While  these  periods  are  not  sharply  marked  off  from  each 
other,  they  do  represent  different  epochs  in  the  development  of 
the  conception  of  the  nature  of  the  cell. 

i.    THE  EARLY  CONCEPTION  OF  THE  CELL   (1839-1861) 

The  Formulation  of  the  Cell  Theory,  1839.  —  It  was  not 
definitely  proved  until  about  1839  that  the  tissues  of  animals 
and  plants  were  composed  of  cells,  although  cells  were  first 
described  in  1665  by  Robert  Hooke.  A  microscopic  study  of  a 
piece  of  cork  showed  him  that  it  was  made  up  of  large  numbers 
of  minute  compartments  which  reminded  him  of  the  cells  of  a 
monastery.  Hence  he  gave  them  the  name  of  cells,  which  they 
still  bear.  Miscellaneous  observations  followed  at  intervals  in 
the  next  two  centuries.  In  1833  Brown  described  the  nucleus 
as  a  constant  part  of  the  cell.  In  the  years  1838  and  1839  two 
Germans,  Schwann  and  Schleiden,  one  studying  animals  and 
the  other,  plants,  advanced  the  theory  that  the  tissues  of  all 
animals  and  plants  were  made  up  of  these  independent  units, 
to  which  they  still  gave  the  name  of  cells.  These  observations 
formulated  the  so-called  cell  doctrine. 

The  Original  Conception  of  the  Cell. — It  was  first  supposed 
that  the  cell  wall  was  the  most  essential  part  of  the  cell  in  con- 
trolling the  processes  of  life  and  separating  die  contents  of  the 
cell  from  the  surrounding  medium.  This  conception  did  not 
last  long,  for  it  was  soon  seen  that  there  were  many  cells  that 
did  not  have  cell  walls.  In  these  early  days  the  existence  of  a 
nucleus  was  not  realized  as  of  much  significance. 

The  Origin  of  Cells. — In  the  beginning  it  was  supposed  that 
cells  were  like  crystals  and  developed  from  a  cytoblastema  as 


CELLS  AND  THE  CELL  THEORY 


39 


crystals  form  in  a  supersaturated  solution  of  sugar,  the  cytoblas- 
tema  being  described  as  a  complex,  supersaturated  solution 
formed  by  the  living  body.  This  theory  did  not  last  many  years, 
however,  because  it  was  shown  that 
cells  arise  only  from  other  cells.  Even 
as  early  as  1846,  Schultze  and  others 
proved  that  cells  have  no  other  origin 
except  from  previously  existing  cells. 
Starting  with  an  egg,  which  is  easily 
demonstrated  to  be  a  single  cell  (Fig. 
15  A),  and  then  carefully  studying  its 
development,  it  can  be  shown  that 
its  growth  is  by  the  method  of  re- 
peated division  and  sub-division  (Fig. 
15  £,  C,  D,  E,  F)  until  the  single- 
celled  egg  gradually  becomes  the 
many-celled  adult.  Although  the 
cells  become  very  numerous,  they  all 
arise  by  the  process  of  division  from 
the  original  egg  cell.  For  many  years, 
however,  it  was  considered  possible 
for  a  cell  to  arise  in  some  other  way 
than  by  division  of  the  original  egg 
cell;  and  even  as  late  as  1880  discus- 
sions took  place  as  to  whether  "free 
cell  origin"  was  possible.  By  this  ghowing  how  &  single.celled  egg 
term  was  meant  the  origin  of  cells  (A),bydiv»onCBtoO),grow»iiito 

a  many-celled  animal. 

from  any  source  except  from  a  previ-  ^  endoderm; 

ously  existing  cell.    In  time  this  ques-  ec' ectoderm- 

,.  ,,1      i    •        ,!  A-  j        F  and  £  show  side  folding  inward 

tlOn  Was  Settled   in   the   negative,  and    to  form  what  becomes  the  digestive 
.     .        . ,  tract. 

we  are  now  certain  that  cells  never 

arise  except  from  the  division  of  earlier  cells,  and  that  all  the 
cells  of  an  adult  animal  body,  though  there  may  be  millions, 
have  arisen  by  the  process  of  division  from  the  original  egg, 
which  was  in  itself  the  single  cell  from  which  the  life  of  the 


en 


F 

FIG.  15. —  THE  DEVELOP- 
MENT OF  THE  EGG  OF  A 
SEA-URCHIN 


40  BTOLOG\ 

individual  started.  Figure  15  shows  how  the  single  cell  divides 
and  continues  to  divide  to  produce  the  many  cells  of  the 
adult  organism. 

2.  PROTOPLASM  AND  THE  MECHANICAL  THEORY  ( 1861-1885 ) 

The  Discovery  of  Protoplasm. — In  1839  Purkinje  first  recog- 
nized under  the  name  "sarcode"  the  contents  of  the  animal  cell; 
H.  Von  Mohl  in  1846  applied  the  term  protoplasm  (Gr.  protos  = 
first  +  plasma  =  substance  or  form)  to  the  viscid,  granular  sub- 
stance found  in  plant  cells.  Cohn  in  1850  claimed  not  only  the 
identity  of  animal  and  plant  protoplasm  but  contended  that  it 
was  the  seat  of  vitality, — the  basis  of  life.  In  1861  Max  Schultze 
established  Cohn's  theory  and  extended  the  meaning  of  the 
word  protoplasm  to  include  all  living  matter.  This  was  a  new 
conception  and  at  once  placed  the  doctrine  of  biology  upon  a 
new  basis.-  If  it  could  be  proved  that  the  cell  substance,  which 
is  the  living  material  in  all  cells,  is  always  alike,  it  would  show 
that  life  could  be  reduced  to  one  fundamental  basis.  The  name 
protoplasm  had  been  given  to  the  living  substance  in  the  animal 
embryo  and  then  to  a  similar  material  in  the  cells  of  plants;  but 
it  was  Schultze  who  identified  it  with  the  living  material  of 
animal  cells  and  extended  the  name  to  apply  to  this  universal 
life  substance.  With  this  new  conception,  he  defined  a  cell  as 
a  mass  of  protoplasm  surrounding  a  nucleus,  and  thus  placed  the 
keystone  in  the  arch  of  the  protoplasmic  theories. 

Schultze's  conception  of  protoplasm  was  somewhat  expanded 
and  made  more  significant  by  Professor  Huxley  in  1866.  Hux- 
ley, giving  to  it  the  name  "physical  basis  of  life,"  drew  far- 
reaching  conclusions  as  to  the  significance  of  the  phenomenon 
that  we  call  life,  based  upon  this  universal  physical  substance. 
He  argued  that  the  properties  of  life  are  simply  characters  of 
this  protoplasmic  substance,  just  as  other  properties  are  char- 
acteristic of  water;  and  that  life  represents  no  distinct  entity, 
but  is  simply  a  name  applied  to  the  combined  properties  of 
this  remarkable  chemical  compound,  protoplasm.  This  started 


CELLS  AND  THE  CELL  THEORY          41 

a  long  search  for  a  chemical  explanation  of  life  phenomena. 
In  accordance  with  this  idea,  life  was  looked  upon  as  merely 
representing  a  special  manifestation  of  chemical  and  physical 
forces;  it  was  argued  that  there  was  no  more  reason  to  speak  of 
vitality  as  a  special  property  possessed  by  living  things,  than  to 
speak  of  aquosity  as  a  special  property  possessed  by  the  chemical 
compound  water. 

The  Mechanical  Theory  of  Life. — Based  upon  this  conception 
arose  a  large  number  of  interesting  speculations,  and  the  discus- 
sions during  the  next  twenty-five  years  resulted  in  a  develop- 
ment of  the  mechanical  theory  of  life.  It  was  argued  that,  if 
life  is  merely  a  name  given  to  the  properties  of  protoplasm,  and  if 
chemists  could  manufacture  the  chemical  substance  protoplasm, 
they  could  thus  create  life,  i.e.,  living  protoplasm.  Chemistry 
was  at  this  time  advancing  with  prodigious  strides,  and  chemists 
were  making  more  and  more  complex  substances,  and  new  com- 
pounds which  had  hitherto  been  considered  beyond  their  reach. 
Many  of  the  substances,  which  had  previously  been  supposed  to 
be  produced  only  by  living  processes,  were,  one  by  one,  manufac- 
tured synthetically  in  the  chemist's  laboratory.  From  this  the 
further  assumption  and  confident  prediction  was  made  that  the 
time  would  come  when  it  would  be  possible  to  manufacture  a  bit 
of  protoplasm  by  purely  chemical  means;  and  then  it  would  fol- 
low, if  the  mechanical  theory  of  life  were  correct,  that  this  bit  of 
protoplasm  would  necessarily  be  alive  and  scientists  would  thus 
be  able  to  manufacture  a  living  thing.  This  was  the  essence  of 
the  mechanical  theory  of  life  which  largely  dominated  discussion 
of  biology  for  a  quarter  of  a  century. 

General  Properties  of  Protoplasm. — With  this  idea  of  pro- 
toplasm as  the  basis  of  life,  a  large  amount  of  study  was  given 
to  this  interesting  material.  Since  it  is  alive,  it  has  of  course 
all  the  properties  of  life.  If  we  look  upon  protoplasm  as  the 
physical  basis  of  life,  we  may  in  one  sense  say  that  its  proper- 
ties are  as  varied  as  are  the  properties  of  living  things,  since 
the  characteristics  of  living  things  are  based  upon  the  charac- 


42  BIOLOGY 

teristics  ot  their  protoplasm.  If  the  characters  of  mankind  are 
dependent  upon  the  properties  of  its  protoplasm,  it  follows 
that  the  protoplasm  that  makes  up  the  cells  in  man  must 
differ  as  much  from  the  protoplasm  that  makes  up  the  cells 
of  a  plant  as  mankind  differs  from  the  plant.  There  will  be, 
then,  is  many  varieties  of  protoplasm  as  there  are  varieties  of 
living  beings  in  the  world.  But  apart  from  these  detailed  charac- 
ters, we  find  that  the  substance  protoplasm,  using  this  term  now 
to  refer  to  the  general  life  substance  of  the  cell,  has  a  few  charac- 
teristics that  are  present  in  all  forms  of  protoplasm  whether 
animal  or  plant.  In  other  words,  all  forms  of  living  matter 
possess  certain  general  properties,  which  are  frequently  spoken  of 
as  the  general  characters  of  protoplasm.  They  are  as  follows : — 
I.  Chemistry  of  Protoplasm. — Various  attempts  were  made 
in  earlier  years  to  determine  the  chemical  composition  of  pro- 
toplasm. The  chemical  elements  out  of  which  it  is  made  are 
easily  found  to  be  carbon,  hydrogen,  oxygen,  nitrogen,  sulphur, 
and  some  other  substances  in  small  quantities.  For  a  time  it 
was  supposed  to  be  a  definite  chemical  substance  with  a  definite 
formula,  and  attempts  were  even  made  to  give  the  number  of 
atoms  present  in  a  molecule  of  protoplasm  We  now  know  that 
such  attempts  were  necessarily  futile.  Protoplasm  is  not  a 
chemical  compound  but  a  mixture  of  a  variety  of  different  com- 
pounds. The  fibrillar  network,  the  liquids,  the  microsomata, 
and  the  chromatin  are  certainly  all  different  from  each  other, 
and  it  is  manifestly  impossible  to  speak  of  the  chemical  composi- 
tion of  protoplasm  as  a  whole.  We  can  safely  say  that  proto- 
plasm contains  proteids,  but  beyond  this,  little  of  significance 
has  yet  been  determined.  Since  it  is  in  a  very  unstable  condi- 
tion, constantly  undergoing  changes,  its  chemical  composition 
cannot  be  constant.  Moreover,  the  chemical  nature  of  living 
protoplasm  is  doubtless  different  from  the  same  material  when 
dead,  and  since  any  chemical  tests  are  sure  to  result  in  its  death, 
it  is  impossible  to  determine  the  composition  of  the  material 
when  alive. 


CELLS  AND  THE  CELL  THEORY          43 

2.  Irritability. — All  forms  of  living  protoplasm  have  the  power 
of  reacting  when  stimulated.    This  phenomenon  is  called  irrita- 
bility and  is  produced  by  the  action  of  a  large  variety  of  external 
ibrces  upon  the  protoplasm  itself.     Any  external  force  which 
serves  to  produce  a  reaction  in  the  protoplasm  is  spoken  of  as 
a  stimulus.     Almost  any  kind  of  stimulus  has  the  power  of 
affecting  protoplasm:  mechanical,  thermal,  electrical,  and  chemi- 
cal.   Stimuli  all  "have  their  effect  upon  protoplasm  and  all  pro- 
duce certain  reactions  within  it.    Protoplasm  is,  in  short,  irri- 
table to  almost  any  external  stimulus.     While  the  different 
forms  of  protoplasm  show  different  degrees  of  irritability  to 
various  stimuli,  they  have  certain  general  reactions  in  common. 
The  activity  of  protoplasm  increases  directly  with  the  heat  to 
a  certain  point,  and  then  decreases,  and  finally  ceases  altogether 
if  the  temperature  continues  to  rise. 

Although  some  forms  of  protoplasm  are  much  more  irritable 
to  mechanical  stimuli  than  others,  nevertheless,  all  types  of  pro- 
toplasm are  influenced  by  external,  mechanical  force.  Various 
other  factors,— light,  chemism,  gravity,  etc.,' — mentioned  upon 
pages  57,  58,  stimulate  protoplasm.  Various  organic,  internal 
changes  stimulate  it  as  well.  If  the  protoplasm  is  improperly 
nourished  it  produces  a  condition  that  is  in  general  known  as 
hunger,  and  this  excites  the  irritability  of  protoplasm.  The  same 
thing  is  true  if  there  is  insufficient  water  within  the  protoplasm, 
producing  an  irritation  called  thirst.  Protoplasm  is  also  destroyed 
by  various  chemicals  called  poisons,  like  chloroform,  corrosive 
sublimate,  etc. 

3.  Conductility. — An  irritation  produced  in  any  one  part  of  a 
bit  of  protoplasm  is  rapidly  conducted  throughout  the  whole 
mass,  a  phenomenon  known  as  conductility.     In  an  ordinary 
cell,  this  phenomenon  of  conductility  does  not  have  very  much 
meaning,  because  the  bit  of  protoplasm  is  too  small;  but  some 
cells   possess   long   protoplasmic   fibers   extending   from   their 
bodies;  and  then  this  function  of  conducting  impulses  from  one 
end  of  the  protoplasm  to  the  other  becomes  of  considerable 


44  BIOLOGY 

importance.  For  instance,  a  nerve  fiber,  even  in  the  higher 
animals,  consists  of  a  long  bit  of  protoplasm  extending  from 
the  cell  body;  see  page  169.  The  phenomenon  of  conductility 
in  this  case  is  of  great  significance  because  it  may  carry  an  im- 
pulse from  the  outer  end  of  these  nerves  (the  periphery)  to  the 
cell  body  in  the  brain,  or  it  may  carry  one  that  started  within 
the  body  rapidly  outward  to  the  periphery.  This  phenomenon 
of  conductility,  therefore,  forms  the  primary  function  of  the 
nerves.  It  is  this  function  that  makes  it  possible  for  a  stimulus 
applied  to  the  outer  part  of  the  animal  to  be  carried  rapidly  over 
the  animal  so  as  to  produce  a  response  in  other  parts  of  the  body. 
4.  Assimilation. — All  protoplasm  has  the  property  of  taking  in 
food  material,  changing  its  chemical  nature  and  converting  it  into 
new  protoplasm  by  assimilation;  a  process  which  may  result  in 
growth.  This  process  is  probably  always  a  constructive  one; 
i.  e.,  it  builds  more  complicated  materials  out  of  simpler  ones. 
Different  kinds  of  protoplasm  have  this  power  developed  to  a 
widely  different  extent.  Some  cells  assimilate  and  grow  with 
great  rapidity,  with  the  result  that  they  multiply  rapidly;  other 
cells  seem  to  have  lost  much  of  this  power  of  assimilation  in  their 
adult  life,  and  are  able  only  to  replace  the  worn-out  parts  of 
their  own  structure.  In  the  higher  animals,  for  example,  the 
cells  are  all  capable  of  rapid  assimilation,  growth,  and  reproduc- 
tion in  youth,  but  many  of  them  nearly  or  wholly  lose  this  power 
after  the  animal  has  reached  adult  life.  The  nerve  cells  in  the 
brain  and  spinal  cord,  for  example,  seem  largely  to  have  lost 
this  property  of  assimilation,  for  they  are  unable  to  grow  after 
they  have  once  reached  the  adult  form,  although  able  to  repair 
their  own  wastes.  Later  in  life,  nearly  all  the  cells  in  the  body 
lose  this  power,  a  condition  characteristic  of  old  age.  Speaking 
generally,  this  power  of  assimilation  and  growth  is  most  active 
at  the  very  beginning  of  the  life  of  a  cell;  it  continues  for  a 
period  with  a  gradually  declining  vigor  and  finally  comes  to  an 
end,  starting  vigorously  again  as  the  result  of  the  process  of 
reproduction. 


CELLS  AND  THE  CELL  THEORY  45 

5.  Reproduction. — Reproduction  is  the  direct  result  of  assimi- 
lation; for  assimilation  produces  growth,  and  growth  in  the  end 
results  in  division.  All  forms  of  reproduction  take  the  form  of 
division. 

The  four  properties,  irritability,  conductility,  assimilation,  and 
reproduction,  have  been  described  as  belonging  to  protoplasm; 
and  the  mechanical  theory  of  life  has  centered  around  this  con- 
ception. But  in  a  sense  it  is  misleading  to  call  them  properties 
of  protoplasm,  unless  in  the  term  protoplasm  we  include  all  of 
the  contents  of  a  cell,  the  nucleus  as  well  as  the  cell  substance. 
A  living  cell  shows  these  general  properties;  but  the  living 
cell  consists  of  protoplasm  and  nucleus,  both  of  which  are  neces- 
sary in  order  that  all  the  functions  mentioned  should  be  shown. 
The  material  frequently  called  protoplasm,  i.  e.,  the  substance 
outside  of  the  nucleus,  does  not  show  all  these  functions.  We 
ask,  therefore :  What  are  the  functions  of  the  nucleus  and  proto- 
plasm as  distinct  from  each  other?  To  draw  a  sharp  line 
between  them  is  not  possible  at  present. 

3.    THE  NUCLEUS  AND  ITS  SIGNIFICANCE    (1880  TO  THE 
PRESENT) 

In  the  early  study  of  the  cell  the  nucleus  was  looked  upon  as 
an  unimportant  part,  and  in  all  of  the  early  discussions  its  sig- 
nificance was  generally  neglected.  From  about  1880  the  modern 
microscope  and  modern  methods  began  to  be  directed  towards 
the  nucleus,  and  a  series  of  marvelous  and  unexpected  results 
were  obtained,  leading  to  the  recognition  of  the  nucleus  as  perhaps 
the  most  important  part  of  the  cell,  and  as  possessing  a  structure 
of  wonderful  complexity  and  marvelous  properties.  The  struc- 
ture of  the  nucleus  has  already  been  outlined  and  may  be  seen  in 
Figure  12.  These  figures  are  enough  to  disprove  any  idea  that 
either  cytoplasm  or  nucleoplasm  can  be  considered  a  definite 
chemical  substance.  They  indicate  clearly  that  in  the  simplest 
life  unit,  we  are  not  dealing  with  a  homogeneous  compound  but 
with  a  complex  structure  and  a  mechanism  of  delicate  adjust- 


46 


BIOLOGY 


ment.  This  has  been  made  even  more  evident  and  brought  to 
a  point  beyond  discussion  by  a  study  of  the  functions  of  the 
nucleus. 

A  nucleus  is  necessary  to  the  complete  life  of  a  cell.  Among 
the  unicellular  animals  are  some  cells  large  enough  for  experi- 
menters to  cut  to  pieces  in  order  to  study  the  different  functions 
of  the  fragments.  These  experiments  are  very  difficult  and  deli- 
cate, but  they  have  been  carried  on  by  a  number  of  investigators 
independently,  who  have  demonstrated  the  following  facts:  If 
a  cell  is  cut  to  pieces  in  such  a  way  that  each  piece  contains  a 
fragment  of  the  nucleus,  ^each  fragment  is  capable  of  carrying 
on  independently  all  life  functions.  Each  can  feed,  grow,  and 


FIG.  16. — STENTOR. 
A  SINGLE-CELLED 
ANIMAL;  n,  THE 

LONG   NUCLEUS 


FIG.  17. — SHOWING  HOW  THE  STENTOR, 
WHEN  CUT  INTO  TWO  PIECES  ALONG 
THE  LINE  AB,  DEVELOPS  INTO  TWO 
COMPLETE  ANIMALS 


multiply,  and  seems  to  be  lacking  in  none  of  the  essential  func- 
tions of  life;  Figs.  16  and  17.  If,  however,  the  animal  is  cut  to 
pieces  in  such  a  way  that  some  of  the  fragments  contain 
pieces  of  the  nucleus,  while  others  contain  none,  the  frag- 


CELLS  AND  THE  CELL  THEORY 


47 


ments  act  in  totally  different  ways.  Those  that  contain  nu- 
clear material  are  able  to  redevelop  lost  parts,  to  carry  on  their 
life  processes  and  to  grow  and  multiply  as  usual;  the  fragments 
that  contain  none  of  the  nucleus,  although  they  can  move  around 
and  apparently  maintain  life  for  a  while,  are  unable  to  feed,  or  at 
least  to  assimilate  their  food ;  they  are 
unable  to  grow  and  unable  to  multiply; 
Fig.  18.  They  have  thus  lost  the  most 
essential  features  of  life,  since  they 
have  lost  the  constructive  power  by 
which  protoplasm  can  assimilate  and 
grow.  These  experiments,  repeated 
many  times  over,  show  that  the  com- 
plete life  of  a  cell  is  impossible  with- 
out the  presence  of  a  certain  amount 
of  nuclear  material,  but  if  nuclear 
matter  is  present,  the  cell  can  carry 
on  its  complete  life,  even  though 
the  nucleus  is  itself  cut  into  many 
pieces.  Such  experiments,  of  course, 
demonstrate  very  conclusively  that 
life  functions  cannot  be  carried  on  by 
protoplasm  alone,  but  only  by  proto- 
plasm in  combination  with  nuclear 
substance. 

The  Nucleus  in  Heredity. — It  is  well  to  anticipate  here  one 
further  fact  that  demonstrates  the  great  significance  of  the 
nucleus  and  chromatin.  As  we  shall  notice  on  a  later  page, 
nearly  all  animals  and  plants  show  a  form  of  reproduction  in 
which  cells  from  two  different  individuals,  male  and  female, 
combine.  This  is  known  as  sexual  reproduction  or  fertilization. 
When  this  union  takes  place,  it  is  not  the  whole  cells  that  com- 
bine but  only  the  nuclei;  or  still  more  accurately,  it  is  the 
chromatin  material  of  the  cells  that  combines  rather  than  the 
whole  nuclei.  The  reconstructed  cell  contains  chromatin  ma- 


FIG.  18. — STYLONYCHIA. 
A  SINGLE-CELLED  ANIMAL 

If  cut  along  the  lines  AB  and 
CD,  only  the  middle  piece  con- 
tains any  nuclear  matter;  this 
alone  develops  into  a  complete 
individual,  the  other  fragments 
soon  dying;  n,  the  two  nuclei. 


48  BIOLOGY 

terial  from  both  of  the  cells  which  entered  into  the  combination. 
Now  inasmuch  as,  after  this  combination,  the  offspring  which 
arises  from  the  cell  thus  formed  by  the  union  of  the  two  parental 
cells  inherits  characteristics  from  both  parents,  and  inasmuch 
as  the  only  part  of  the  original  sex  cells  which  enters  into  the 
union  is  the  chromatin,  it  follows  that  the  chromatin  material 
itself  is  the  bearer  of  heredity,  and  that  in  these  little  chromatin 
threads,  minute  as  they  are,  there  must  be  a  complexity  suffi- 
cient to  contain  the  features  of  inheritance  that  are  handed  on 
from  generation  to  generation. 

These  facts  give  at  least  some  idea  of  the  separate  properties 
of  cell  substance  and  nucleus.  The  cell  substance  by  itself  has 
the  functions  of  irritability  and  conductility ;  but  not  of  assimi- 
lation, growth,  or  reproduction.  These  latter  functions  can  be 
carried  on  only  when  a  nucleus  is  present. 

WHAT  IS  MEANT  BY  PROTOPLASM 

It  has  become  evident  by  this  time  that  the  original  con- 
ception of  protoplasm  has  quite  disappeared.  Indeed,  if  we 
ask  to-day  just  what  is  meant  by  protoplasm,  the  question 
becomes  very  difficult  to  answer.  We  can  no  longer  look  upon 
it  as  simply  the  jelly-like  substance  within  the  cell  in  which  the 
nucleus  lies  embedded,  for  it  is  evident  that  although  this 
substance  has  the  properties  of  irritability  and  conductility,  it 
does  not  have  the  properties  of  assimilation  and  growth.  If  we 
wish  still  to  call  protoplasm  the  physical  basis  of  life,  we  must 
extend  the  term  to  include  the  nucleus  as  well  as  the  sub- 
stance outside  of  the  nucleus,  since  without  the  nucleus, 
protoplasm  is  unable  to  carry  on  life  processes.  If,  however, 
we  include,  in  this  term  protoplasm,  the  centrosome,  and  the 
nucleus  with  its  chromosomes,  it  becomes  evident  that  proto- 
plasm has  quite  lost  its  original  significance.  It  is  no  longer 
the  homogeneous  substance,  and  can  no  longer  be  looked  upon 
as  a  chemical  compound,  but  is  on  the  other  hand  a  mechanism 
with  a  number  of  distinct,  though  closely  correlated  parts. 


CELLS  AND  THE  CELL  THEORY  49 

The  explanation  of  its  activities  can  no  longer  be  regarded  as  a 
chemical  problem  simply,  but  must  be  in  a  measure  a  mechanical 
problem  as  well.  This  conception  totally  alters  the  significance 
of  the  phrase  "the  physical  basis  of  life"  and  puts  the  prob- 
lem of  the  mechanical  theory  upon  a  decidedly  new  footing. 

To-day  biologists  are  gradually  giving  up  the  use  of  the  term 
protoplasm  as  confusing  and  misleading,  replacing  it  by  more 
definite  terms  which  refer  directly  to  the  different  parts  of  the 
cell.  So  now  we  find  coming  into  general  use  the  terms  cytoplasm 
and  karyoplasm  (see  page  32)  to  cover  what  was  formerly  called 
protoplasm.  Both  cytoplasm  and  karyoplasm  are  necessary 
and  must  act  together  in  order  to  show  the  general  characters 
of  life.  That  reproduction  may  occur,  the  chromatin,  and  per- 
haps the  centrosome  also,  are  requisite. 

The  mechanical  theory  is  no  longer  tenable  in  the  form  in 
which  it  was  originally  advocated  and  discussed.  That  position 
has  been  necessarily  abandoned  since  the  studies  of  more  recent 
years  have  demonstrated  that  protoplasm  is  not  a  homogeneous 
substance  and  cannot  be  regarded  simply  as  a  chemical  com- 
pound. It  is,  on  the  contrary,  a  very  complex  mixture  of  sub- 
stances, forming  a  complicated  machine  in  which  the  parts  are 
most  intricately  interrelated  and  adjusted.  While  chemical  forces 
may  be  regarded  as  sufficient  to  manufacture  almost  anything 
in  the  way  of  chemical  compounds,  they  are  not  adapted  to  the 
manufacture  of  such  a  mechanism  as  living  protoplasm  has 
been  proved  to  be.  This  change  in  the  attitude  of  biologists 
has  been  brought  about  mainly  through  the  minute  study  of 
the  nucleus  and  the  constantly  increasing  recognition  of  its 
great  importance  in  the  life  of  the  cell. 

Are  There  Life  Units  Simpler  Than  Cells?— As  we  have 
learned,  the  cell  is  by  no  means  a  simple  structure  but  a  compli- 
cated mechanism.  The  question  inevitably  arises  whether  the 
cell  is  the  simplest  structure  that  can  manifest  life  or  whether  it 
may  not  be  analyzed  into  simpler  units.  This  is  one  of  the 
puzzling  and  unsettled  problems  of  biology.  Certainly  some  of 


50  BIOLOGY 

the  most  minute  living  things  (certain  bacteria)  seem  to  possess 
a  body  in  which  there  is  no  definite  nucleus,  but  in  which  the 
chromatin  matter  is  more  or  less  scattered  without  being  aggre- 
gated into  a  nuclear  mass,  and  this  has  led  to  the  suggestion 
that  perhaps  the  simplest  life  unit  may  be  an  excessively  minute 
granule  of  chromatin  with  delicate  fibrils  extending  from  it,  and 
that  a  cell  is  a  combination  of  many  of  these  minute  elements. 
Other  facts  disclosed  by  the  minute  study  of  many  animal  cells, 
with  very  high  magnifying  powers  and  under  special  conditions, 
have  pointed  to  a  similar  conclusion.  As  a  result  there  has  been 
advanced  recently  a  theory  that  the  cell  is  far  from  the  simplest 
unit  of  life,  and  that  it  can  be  analyzed  into  a  great  number  of 
minute  elements  called  "chromidial  units,"  each  made  of  a 
granule  of  chromatin  with  fibers  of  linin  radiating  from  it. 
According  to  this  theory  the  whole  cell  is  made  of  a  network 
of  linin  fibers  with  granules  at  the  nodes,  each  granule  thus 
representing  a  life  unit  far  simpler  than  a  cell.  This  has  been 
called  the  "protomitomic  network."  This  protomitomic  theory 
is  as  yet  only  a  matter  of  speculation,  and  its  chief  interest 
to-day  is  in  the  fact  that  it  suggests  that  the  cell  may  be  far 
from  the  simplest  unit  manifesting  life.  Whether  this  new 
suggestion  be  established  or  not,  it  seems  certain  that  the 
manifestation  of  life  requires  the  presence  of  three  elements: 
(1)  chromatin  material,  (2)  delicate  fibrils  radiating  from  it,  and 
(3)  of  a  liquid  material  in  which  the  other  parts  are  embedded. 
As  yet  we  know  of  nothing  simpler  than  a  combination  of  these 
three  that  is  able  to  manifest  all  the  properties  of  life. 


LABORATORY  WORK  ON  CELLS 

A  satisfactory  study  of  cells  requires  familiarity  with  the  microscope  and 
considerable  skill  in  microscopic  methods.  Little  can  be  wisely  undertaken 
by  elementary  students,  beyond  the  examination  of  prepared  specimens, 
properly  stained,  which  should  be  furnished  by  the  instructor.  Drawings 
should  be  made  by  the  student  in  all  cases.  The  cellular  structure  of  animal 
tissues  may  be  studied  in  the  following  preparations:  — 

Blood. — A  small  drop  of  frog's  blood  in  a  little  normal  solution  (.9% 


CELLS  AND  THE  CELL  THEORY          51 

NaCL)  examined  with  a  1/6  inch  objective,  will  show  blood  cells,  the  red 
cells  having  nucleii. 

Cartilage. — Mounted  sections  of  cartilage  will  show  nearly  rounded  cells, 
embedded  in  a  very  thick  mass  of  cell  wall,  the  thickened  cell  wall  forming 
the  intercellular  substance,  or  basis  of  the  cartilage. 

Bone. — Mounted  sections  will  show  cells  lying  in  irregular  spaces,  within 
a  hard  secreted  mass  of  intercellular  substance  in  which  mineral  salts  have 
been  deposited. 

The  cellular  structure  of  plants  may  be  studied  by  the  following  prepara- 
tions:— 

Cork  or  wood  sections  show  plant  tissue  made  of  numerous  cells  of  varying 
shape.  In  these  sections  the  cell  walls  only  appear. 

A  section  of  a  growing  root  tip.  Longitudinal  sections  of  Podophyllum, 
which  are  particularly  good,  should  be  furnished.  These  sections,  if  properly 
stained,  will  show  the  cell  contents  as  well  as  the  cell  walls.  The  protoplasm 
and  nucleus  may  be  seen  and  drawn.  In  particularly  good  specimens, 
stained  with  iron  haematoxylin,  the  chromatin  in  the  nucleus  may  be  seen 
with  an  oil  immersion,  1/12  inch  objective. 

For  the  study  of  protoplasm  Spirogyra  is  a  favorable  object.  The  student, 
after  studying  the  normal  specimen,  should  treat  it  with  a  little  glycerine, 
which  will  cause  the  protoplasm  to  shrink  away  from  the  cell  wall  so  that 
it  can  be  seen. 

The  movement  of  the  protoplasm  within  the  cell  is  best  seen  in  the  long 
internodal  cells  of  Chara  or  Nitella.  It  may  also  be  seen  in  the  stamen  hairs 
of  Tradescantia. 

Ci'iary  motion  may  be  studied  best  by  cutting  off  a  bit  of  the  edge  of  the 
gill  of  a  fresh-water  clam,  and  examining  with  a  high-power  objective.  It 
may  also  be  shown  by  scraping  the  roof  of  a  frog's  mouth  with  a  scalpel 
and  mounting  the  scrapings  in  a  little  normal  fluid. 

BOOKS  FOR  REFERENCE 

WILSON,  The  Cell  in  Development  and  Inheritance,  Macmillan  Co., 
New  York. 

BAILEY,  Text-book  of  Histology,  Wm.  Wood,  Philadelphia,  Pa. 

STOHR,  Text-book  of  Histology,  P.  Blakiston's  Son,  Philadelphia,  Pa. 

DAHLGREN  and  KEENER,  Principles  of  Animal  Histology,  Macmillan 
Co.,  New  York. 

MELL,  Biological  Laboratory  Methods,  Macmillan  Co.,  New  York. 

HERTWIG,  Die  Zelle  und  Die  Gewerbe,  Gustav.  Fischer,  Jena. 

CALKINS,  Protozoology,  Lea  and  Febiger,  Philadelphia,  Pa. 

BERNARD,  Some  Neglected  Factors  in  Evolution,  G.  P.  Putnam's  Sons, 
New  York. 


CHAPTER  III 

UNICELLULAR  ORGANISMS 

IN  order  to  become  familiar  with  the  general  properties  of 
living  things,  we  will  study  the  structure  and  functions  of  some 
of  the  simplest  organisms.  Those  that  are  studied  in  this  chap- 
ter are  all  microscopic,  and  belong  to  the  group  of  unicellular 
organisms  sometimes  called  animalculae. 

ANIMALS 

The  first  organisms  to  be  studied  are  undoubtedly  to  be 

regarded  as  animals. 

AM(EBA 

Size  and  Shape. — The  Amoeba  (Gr.  amoibos  —  changing)  is 
a  microscopic  animal  found  both  in  fresh  and  salt  water.  The 
most  common  species  averages  about  1/100  of  an  inch  in  diam- 
eter, but  the  size  varies  in  different  species.  With  perseverance 
they  may  be  discovered  in  nearly  all  bodies  of  water  where  there 
is  mud  and  slime.  One  of  the  best  methods  of  procuring  them 
for  study  is  to  collect  water  plants  (Ceratophyllum)  or  even  pond- 
lily  leaves,  and  to  place  them  in  dishes  of  water  until  they  decay. 
After  a  couple  of  weeks  or  so  a  brown  scum  appears  and  an 
examination  of  this  scum  usually  shows  Amoebce  in  abundance. 

Under  the  microscope  the  Amoeba  is  seen  to  be  a  single  cell 
without  definite  form,  the  same  animal  undergoing  constant 
changes  in  outline.  Lobes  are  thrust  out  first  in  one  direction 
and  then  in  another  (Fig.  19),  and  as  soon  as  one  lobe  is  protruded 
the  contents  of  the  body  begin  to  flow  into  it  and  may  continue 
to  flow  until  the  whole  body  substance  has  passed  into  the  lobe, 
other  lobes  being  formed  in  the  meantime.  By  a  continual  pro- 
trusion of  such  lobes  and  the  flowing  of  the  body  into  them,  the 
Amoeba  has  a  slow  motion.  These  lobes  are  thus  used  as  organs 
of  locomotion  and  are  called  pseudopodia  (Gr.  pseudos  =  false  -f 
pous  =  foot). 

52 


UNICELLULAR  ORGANISMS 


53 


There  has  been  considerable  speculation  as  to  the  forces  which 
produce  pseudopodia,  and  various  attempts  have  been  made  to 
explain  them  by  purely  physical  forces.  It  has  been  suggested 
that  they  are  due  to  the  adhesion  of  the  sticky  substance  of 
which  the  animal  is  made,  to  the  object  upon  which  it  rests. 


ec 


FIG.  19. —  AMCEBA  PROTEUS 

A,  the  animal  in  its  natural  condition;  B,  an  animal  that  has  swallowed  a  long  filamentous 
plant;  C,  the  animal  in  the  state  of  division. 

cv,  contractile  vacuole;  ex,  remains  of  undigested  food; 

ec,  ectoplasm;  p,  protoplasm. 

en,  endoplasm; 

Another  suggestion  is,  that  the  pseudopodia  are  due  to  changes 
in  surface  tension  produced  by  the  currents  in  the  body  as  they 
flow  to  and  fro.  Still  another  theory  seeks  to  explain  the  forma- 
tion of  pseudopodia  by  stereotropism  (Gr.  stereos  =  a  solid  + 
trope  =a  turning),  the  attraction  of  a  solid  body  for  living 
tissue,  which  is  supposed  to  cause  the  body  of  the  animal  to 
flow  from  one  point  to  another  of  the  surface  upon  which  it 
rests.  There  is  also  the  theory  of  chemical  attraction. 


54  BIOLOGY 

However,  the  production  of  these  pseudopodia  cannot  be 
satisfactorily  explained  by  any  of  these  means;  enough  careful 
study  of  the  Amceba  in  motion  has  been  made  to  show  that  the 
pseudopodia  may  be  thrust  out  in  any  direction,  either  horizon- 
tally or  vertically;  and  when  thrust  out  vertically  they  may  be 
bent  forward  until  they  come  in  contact  with  the  surface  on 
which  the  animal  rests  and  then  become  attached.  Their  motion 
has  to  be  explained  by  an  active  power  of  the  living  substance. 
This  power  on  the  part  of  the  living  substance  has  been  called 
contractility,  and  it  cannot  be  explained  as  due  to  any  physical 
force  like  surface  tension,  adhesion,  or  chemical  attraction,  but 
is  due  rather  to  active  contraction  which  must  be  regarded  as 
a  general  function  of  the  protoplasm  of  a  living  cell. 

Structure. — The  body  of  Amoeba  is  made  up  of  a  transparent 
mass  of  protoplasm,  in  which  there  may  be  distinguished  an 
outer  clearer  layer,  called  ectoplasm  (Gr.  edos  =  outside  + 
plasma),  and  an  inner,  more  granular  mass  called  endoplasm 
(Gr.  endon  =  within  +  plasma).  No  very  definite  line  can  be 
drawn  between  them,  the  difference  being  due  chiefly  to  the 
presence  of  granules  in  the  interior  and  their  absence  from  the 
outer  layer.  These  granules  are  in  motion,  slowly  circulating 
within  the  animal,  and  thus  showing  the  existence  of  currents 
in  the  protoplasm.  When  the  pseudopodia  are  protruded,  the 
first  change  is  the  protrusion  of  a  lobe  of  the  ectoplasm;  after 
which  the  granules  can  be  seen  flowing  into  the  lobe  until 
finally  the  whole  of  the  endoplasm  may  flow  into  the  extruded 
lobe.  Many  of  these  granules  represent  food  in  various  stages 
of  digestion,  some  of  them  being  digested  food  and  others  un- 
digested refuse.  Among  them  may  be  found  drops  of  clear 
liquid  with  a  bit  of  digested  food  in  their  center. 

Besides  these  granules,  two  more  definite  bodies  are  always 
found.  One  (Fig.  19  ri),  the  nucleus,  is  a  small  rounded  body 
near  the  center  of  the  animal,  but  not  fixed  in  position,  since  it 
moves  with  the  protoplasmic  current.  This  is  one  of  the  struc- 
tural parts  of  the  animal,  not,  like  most  of  the  granules,  merely 


UNICELLULAR  ORGANISMS  55 

extraneous  material,  and  is  always  present  in  the  living  animal. 
The  other  body  commonly  found  is  the  contractile  vacuole  (Lat. 
vacuus  =  empty)  (Fig.  19  cv).  This  is  a  clear,  pulsating  drop, 
at  one  moment  appearing  as  a  good-sized  sphere,  and  the  next 
contracting  and  disappearing,  to  reappear  again.  It  is  thought 
that  when  it  contracts,  its  contents,  which  are  liquid,  are  forced 
out  of  the  Amoeba's  body  through  minute  openings  that  appear 
in  its  sides.  These  pulsations,  which  are  fairly  regular,  plainly 
indicate  the  performance  of  some  important  function. 

Assimilation  and  Growth. — When  the  Amceba  comes  in  con- 
tact with  a  small  plant  or  other  bit  of  food,  the  pseudopodia 
flow  around  and  over  it  so  that  the  food  is  taken  bodily  inside 
the  animal.  The  food  may  be  taken  in  at  any  point  on  the 
surface  of  the  Amoeba's  body,  though  more  frequently  it  is 
engulfed  by  the  anterior  pseudopodia.  As  shown  in  Fig.  19  B, 
particles  of  food  longer  than  the  whole  animal  may  be  ingested. 
After  a  time  the  bit  of  food  thus  ingested  begins  to  show  signs 
of  disintegration.  It  loses  its  sharp  outline  and  becomes  slowly 
softened  and  dissolved.  This  change  is  produced  by  the  action 
of  certain  fluids  which  the  animal  secretes,  and  is  a  process  of 
digestion.  The  nutritious  portions  become  in  time  absorbed 
by  the  protoplasm  and  converted  into  new  Amoeba  substance; 
the  last  process  being  assimilation.  The  refuse  finds  its  way 
eventually  to  the  surface  of  the  animal,  a  temporary  opening- 
appears  and  the  Amoeba  crawls  away,  leaving  the  refuse  behind 
it;  Fig.  19  ex.  Any  part  of  the  body  may  thus  serve  for  the 
ingestion  of  food  or  the  ejection  of  refuse,  although  the  food  is 
commonly  taken  in  at  the  anterior  end,  and  the  refuse  ejected 
from  the  posterior  end. 

Respiration. — Amoeba  is  not  only  carrying  on  a  process  of 
assimilation,  by  which  new  substances  are  built  up,  but  is  also 
at  the  same  time  carrying  on  a  process  of  disintegration,  by 
which  the  complex  substances  are  broken  down.  This  latter 
is  based  upon  oxidation  or  union  with  oxygen.  As  the  result  of 
oxidation  there  is  always  formed  carbon  dioxid  gas  (C02)  as  a 


5C  BIOLOGY 

waste  product,  which  must  be  eliminated.  The  Amoeba  is, 
therefore,  obliged  to  absorb  oxygen  gas  from  some  source  and 
to  eliminate  carbon  dioxid  gas.  This  process  of  absorbing  and 
eliminating  gases  is  known  as  respiration.  In  the  Amoeba  there 
appear  to  be  no  special  respiratory  organs,  although  possibly 
the  contractile  vacuole  performs  this  function.  But  the  body 
of  the  animal  is  so  small  that  special  respiratory  organs  are 
unnecessary,  since  gas  is  readily  absorbed  directly  through  the 
surface  of  the  body  from  the  water  in  which  the  animal  lives, 
and  carbon  dioxid  is  as  readily  eliminated  into  the  water.  A 
respiratory  function  is  thus  developed,  but  no  distinct  respira- 
tory organs.  The  elimination  of  carbon  dioxid  gas,  since  it  is 
the  getting  rid  of  a  waste  product  of  metabolism,  is  not  only 
part  of  the  function  of  respiration,  but  belongs  also  to  the  func- 
tion of  excretion. 

Excretion. — As  the  result  of  this  disintegration  there  arise 
in  the  Amoeba  disintegration  products  which  are  waste  materials 
and  must  be  eliminated  from  the  body.  These  products  are 
primarily  three :  carbon  dioxid  gas,  water,  and  a  product  contain- 
ing nitrogen,  and  related  to  urea  which  is  excreted  by  the  kid- 
neys of  higher  animals.  The  function  of  getting  rid  of  these 
waste  products  is  called  excretion.  In  Amoeba  the  gas  and 
the  water  are  •  excreted  directly  into  the  surrounding  water, 
either  through  the  general  surface  of  the  body  or  by  the  contrac- 
tile vacuole.  The  urea  is  probably  eliminated  by  the  contractile 
vacuole. 

It  should  be  clearly  recognized  that  the  elimination  of  the  un- 
digested portions  of  the  food,  mentioned  on  page  55,  is  not 
excretion.  These  undigested  parts  of  the  food,  though  sometimes 
called  "excreta,"  have  never  become  part  of  the  Amoeba's  body 
and  are  simply  foreign  bodies  that  have  been  rejected  as  useless. 
True  excretion,  on  the  other  hand,  always  refers  to  the  elimina- 
tion of  the  products  of  dissimilation. 

Relation  to  Water. — Protoplasm  requires  water  for  its  activi- 
ties. Ordinary  active  living  matter  contains  60%  to  80%  of 


UNICELLULAR  ORGANISMS  57 

water,  and  some  forms  of  protoplasm  much  more,  certain 
organisms  containing  over  95%.  When  dormant,  protoplasm 
may  remain  alive  with  a  far  smaller  percentage,  dried  seeds 
containing  as  little  as  8%.  Some  animals  also  may  be  dried 
(dessicated)  and  still  retain  their  vitality  for  a  long  time.  This 
is  true  of  many  of  the  microscopic,  unicellular  animals  and  also 
of  some  of  the  higher  types  (e.  g.,  Hydatina;  see  Fig.  116).  In 
all  such  cases  life  activities  are  suspended  but  will  be  resumed 
when  the  animal  imbibes  water. 

Irritability. — The  Amoeba  has  no  sense  organs  nor  does  it 
have  any  nervous  system.  It  is  difficult  or  impossible  to  deter- 
mine positively  whether  it  has  any  conscious  sensations,  but 
it  certainly  has  the  power  of  reacting  when  stimulated,  thus 
showing  that  it  possesses  irritability. 

Reaction  to  contact  (Thigmotropism)  (Gr.  thigma  =  touch  + 
trope  =  a  turning). — If  the  moving  Amoeba  is  touched  by  a  solid 
object,  the  part  touched  draws  away  from  the  object,  new 
pseudopodia  being  thrust' out  in  another  direction.  If,  however, 
the  object  be  a  particle  of  food,  the  animal  is  differently  affected 
and  the  pseudopodia  flow  around  it  so  as  to  engulf  it. 

Reaction  to  chemicals  (Chemotropism)  (Gr.  chemesa  =  chemis- 
try +  trope ) . —  If  certain  chemicals  are  brought  in  contact 
with  the  Amoeba,  it  moves  off  in  some  other  direction.  Sugar, 
lactic  acid,  sodium  chloride,  and  many  other  substances  have 
this  effect. 

Reaction  to  heat  (Thermotropism)  (Gr.  thermos  =  heat  -{-trope). 
—The  activities  of  the  Amoeba  are  directly  dependent  upon  tem- 
perature. At  a  temperature  of  freezing,  no  activities  are  mani- 
fest. If  the  temperature  is  raised  the  activities  begin  and  become 
more  active  with  the  increase  in  temperature  up  to  a  certain 
point,  about  85°  F.  If  warmed  still  more,  they  become  less 
active,  and  when  heated  to  about  90°  F.  the  activities  cease  en- 
tirely. At  about  105°  F.  the  protoplasm  is  coagulated  and  the 
animal  killed.  If  a  warm  or  hot  object  is  brought  near  an 
active  Amosba  the  animal  moves  away  from  it. 


68  BIOLOGY 

Reaction  to  light  (Phototropism)  (Gr.  photos  -  light  -f-  trope).— 
If  a  strong  light  is  directed  upon  an  Amoeba  from  one  side,  it 
will  move  away  from  the  light.  A  strong,  white  light  may  cause 
the  animal  to  stop  moving. 

Reaction  to  electricity  (Electropism)  (Eng.  electro  -f-  Gr.  trope).— 
If  an  electric  current  is  passed  through  an  Amoeba,  it  contracts 
on  the  side  of  the  positive  pole  of  the  current  and  moves  toward 
the  negative  pole. 

In  all  these  cases  the  Amceba  reacts  to  a  stimulus.  But  there 
are  other  things  which  are  irritable  and  react  to  a  stimulus  in 
a  purely  mechanical  fashion.  Gunpowder  is  also  irritable,  since 
it  will  react  to  heat  with  an  explosion.  A  locomotive  is  irri- 
table, since  it  will  react  to  a  touch  upon  its  throttle  valve.  The 
Amoeba  certainly  reacts  in  a  more  complex  and  more  varied 
manner,  but  the  question  inevitably  arises  whether  the  action 
may  not  be  simply  that  of  a  bit  of  machinery  responding  to  its 
appropriate  stimulus.  There  is  no  definite  answer  to  this  ques- 
tion that  can  yet  be  given. 

Reproduction. — As  the  Amoeba  by  assimilation  converts  its 
food  into  new  protoplasm,  it  inevitably  increases  in  size.  If 
this  went  on  without  interruption  there  would  be  no  limit  to 
the  size  of  the  animal.  But  after  growing  for  a  time,  a  constric- 
tion appears  in  the  middle  of  the  body  which  deepens  until  it 
finally  divides  the  animal  into  two  parts;  Fig.  19  C.  Each  -of 
the  resulting  parts  is  like  the  other  and  each  like  the  original, 
except  in  size.  It  is  the  nucleus  that  seems  to  take  the  lead  in 
this  process  of  division,  which  is  one  of  great  complexity.  This 
will  be  described  in  the  next  chapter,  for  it  goes  through  the 
complicated  series  of  changes  known  as  karyokinesis  (Gr. 
karyon  =  nucleus  +  kinesis  =  movement)  described  on  page  85. 
As  a  result  of  this  division  there  arise  two  animals,  evidently 
alike,  each  of  which  now  moves  away  and  lives  an  independent 
life.  This  method  of  reproduction,  by  which  the  animal  divides 
into  two  practically  equal  parts,  is  called  fission. 

A    second    method    of    reproduction    sometimes    occurs    in 


UNICELLULAR  ORGANISMS 


59 


Amoeba.  This  is  very  unusual,  however,  and  has  been  seen  by 
only  one  observer  (Sheel).  In  this  method  the  animal  draws  in 
its  pseudopodia,  assumes  a  spherical  form  and  secretes  around 
itself  a  thin  shell  called  a  cyst.  Inside  this  cyst  the  nucleus 
divides  into  many  parts,  some  five  or  six  hundred  nuclei  thus 
finally  arising  by  division.  After  this  the  rest  of  the  substance 
divides  so  that  each  nucleus  finally  becomes  surrounded  by  a 
little  protoplasm,  the  contents  of  the  cyst  coming  thus  to  con- 
sist of  some  hundreds  of  little  bodies,  each  with  its  nucleus. 
Eventually  the  cyst  bursts  and  the  little  cells  escape,  each  being 
now  a  minute  Amoeba,  which  has  only  to  grow,  to  be  like  the 
original.  This  method  of  reproduction  is  also  evidently  a  divi- 
sion. It  is  a  type  of  division  called  spore  formation.  The  whole 
process  takes  two  and  a  half  to  three  months,  and  the  condi- 
tions which  bring  it  about  are  unknown. 


FIG.  20. —  SINGLE-CELLED  ANIMALS  RELATED  TO  AKKEBA 

A,  Difflugia,  an  Ama>ba-\ike  animal  with  a  shell  made  of  pebbles;  B  and  C,  Podophrya 
and  Acineta,  animals  with  stiff  protruding  tentacles  of  protoplasm;  /,  food;  D,  Arcella,  an 
Amoeba-like  animal  with  a  secreted  shell. 


PARAMBCIUM 


Paramedum  can  usually  be  found  in  the  same  localities  as 
Amoeba  and  can  easily  be  obtained  by  allowing  lily  pads  to 
decay  in  a  dish  of  water.  A  quantity  of  living  organisms  soon 


60 


BIOLOGY 


appears  in  the  scum  that  forms  on  the  surface,  and  among  them 

may  be  seen  some  minute  white  specks,  just  visible  to  the  naked 

eye,  each  one  of  which  is  a  Parame- 
dum;  Fig.  21. 

Like  the  Amoeba,  it  is  a  single  cell, 
and  like  the  Amoeba  also,  it  is  made 
up  of  protoplasm  consisting  of  an 
outer,  somewhat  clear  ectoplasm  and 
an  inner,  more  granular  endoplasm. 
The  Paramedum  has  a  body  which, 
although  flexible,  is  somewhat  rigid 
and  elastic,  and,  unlike  the  Amoeba, 
always  tends  to  preserve  a  definite 
form.  It  is  elongated,  somewhat 
blunter  at  one  end  than  the  other, 
and  in  its  motion  carries  the  blunt 
end  forward.  The  protoplasm  has 
no  power  of  protruding  pseudopodia, 
and  the  animal  therefore  does  not 
change  its  shape  like  the  Amoeba. 
Upon  one  side,  posterior  to  the 
middle  of  the  body,  there  is  a  groove 
extending  obliquely  backward.  This 
is  the  oral  groove  (og),  at  the  bottom 
of  which  there  is  an  opening  leading 
to  a  short  tube  which  extends  through 
the  ectoplasm  into  the  endoplasm. 
The  opening  is  the  mouth,  and  the 
tube  is  known  as  the  oesophagus  or 
gullet;  oe. 

Locomotion. — The  whole  of  the  outer 
surface  of  the  animal  is  covered  with 
numerous,  fine,  threadlike  projections 

of  protoplasm,  protruding  from  the  ectoplasm  into  the  water. 

These  are  called  cilia  and  are  capable  of  rapid  motion  back 


CV 


ex 


FIG.  21. —  PARAMECIUM 

AURELIA 

ci,  cilia; 

cv,  contractile  vacuole; 

ex,  excreta; 

m,  mouth; 

mic,  micronucleus ; 

mn,  macronucleus; 

mb,  membranella; 

oe,  03sophagus; 

og,  oral  groove. 


UNICELLULAR  ORGANISMS 


61 


and  forth.  Ordinarily  in  life,  they  are  directed  somewhat  back- 
wards, and  as  a  result  of  this  position,  when  they  beat  back  and 
forth  they  cause  the  propulsion  of  the  animal  forward  through 
the  water  with  a  uniform  motion.  When  the  cilia  are  directed 
forward,  their  beating  back  and  forth  will  cause  the  animal  to 
move  backward.  At  the  same  time  with  their  back-and-forth 
motion  they  beat  slightty  to  one  side,  causing  the  animal  to 
rotate  slowly  on  its  long  axis  as  it  moves  either  forward  or  back- 
ward. Exactly  how  these  cilia  are  able  to  move  is  not  known, 
but  a  power  of  automatic  vibration  is  always  characteristic  of 
these  organs.  Lining  the  tube  called  the  oesophagus,  leading 
from  the  mouth,  there  are  special  cilia,  longer  than  the  rest  and 
united  to  form  a  vibrating  membrane  known  as  membranella ; 
Fig.  21  mb.  The  function  of  this  mass  of  fused  cilia  is  to  guide 
the  food  from  the  mouth  down  through  the  oesophagus  into  the 
body  cavity.  The  direction  in  which  the  cilia  point,  and  con- 
sequently the  direction  of  the  motion  they  produce,  are  affected 
by  a  variety  of  external  condi- 
tions, for  the  Paramecium,  like 
the  Amoeba,  is  irritable  and  its 
motions  are  regulated  by  the 
surrounding  conditions. 

Structure. — The  ectoplasm  of 
the  Paramecium  is  somewhat 
clearer  than  the  endoplasm, 
but  it  contains  large  numbers 
of  minute  threadlike  organs 
known  as  trichocysts  (Gr.  trix 
=  hair  +  cystis  =  bag);  Fig. 
22  tr.  These  may  be  discharged 
from  the  animal;  and  they  ap- 
pear to  be  organs  of  offense  or 

defense,  since  they  apparently  contain  a  small  quantity  of  poison 
by  which  the  animal  may  kill  or  paralyze  its  prey  or  its  enemies. 
On  the  very  outside  of  the  ectoplasm  is  an  extremely  thin  mem- 


FlG.  22. —  A  BIT  OF  THE  OUTER 
EDGE  OF  THE  PARAMECIUM 
(HIGHLY  MAGNIFIED) 

(Modified  from  Maier.) 


CM,  cuticle; 
ec,  ectoplasm; 
en,  endoplasm; 


fv,  food  vacuole; 
tr,  trichocyst. 


62  BIOLOGY 

brane  known  as  the  cuticle  (cu),  through  which  the  cilia  pro- 
trude. This  is  ordinarily  invisible  and  can  only  be  seen  under 
special  conditions.  It  is  a  protective  covering  which  makes  the 
body  a  little  more  resistant  than  it  otherwise  would  be.  The 
endoplasm  fills  the  rest  of  the  body  and  is  very  highly  granular, 
containing  large  numbers  of  food  masses  in  various  stages  of  di- 
gestion. The  nucleus  is  double,  showing  a  large  macronucleus* 
(Fig.  21  win),  and  near  it  a  small  micronucleusf,  mic.  These 
two  bodies  lie  close  together  near  the  mouth  and  hold  fairly 
constantly  their  relative  positions  in  the  body  of  the  animal. 
Two  contractile  vacuoles  (cv)  are  found  in  the  common  species 
of  Paramedum,  one  at  each  end.  These  vacuoles  connect 
with  the  different  parts  of  the  body  by  a  number  of  minute 
radiating  canals,  six  or  ten  in  number,  which  extend  in  all  direc- 
tions. Certain  liquids  are,  apparently,  poured  into  these  canals 
from  the  living  protoplasm  and  through  them  flow  into  the 
vacuoles,  which  increase  in  size  until  they  reach  a  certain  mag- 
nitude and  then  suddenly  contract  and  discharge  their  contents 
to  the  exterior,  probably  through  minute  openings.  The  con- 
traction of  the  vacuoles  is  fairly  regular,  varying  in  rapidity 
with  the  temperature;  the  two  vacuoles  do  not  contract  simul- 
taneously, but  alternate  with  each  other.  These  organs,  as  in 
the  case  of  the  Amoeba,  are  probably  associated  with  the  func- 
tion of  respiration  and  excretion. 

Assimilation  and  Growth. — The  food  of  the  Paramedum  con- 
sists chiefly  of  minute  bacteria.  These  are  driven  into  the 
mouth  by  the  action  of  the  cilia,  and  by  the  membranella  in  the 
ossophagus,  and  then  guided  down  the  oesophagus  to  its  inner 
end.  Here  the  bacteria  collect  in  a  little  drop  of  water.  The 
oesophagus  then  contracts  and  pinches  off  this  little  drop  con- 
taining the  bacteria,  and  thus  forms  what  is  called  a  food  vacu- 
ole,  which  enters  into  the  general  mass  of  the  endoplasm  and 
follows  the  movement  of  the  protoplasm  around  the  body. 
The  digestive  juices  are  secreted  and  gradually  digest  the  bac- 
*  Gr.  macros  =  large.  f  Gr.  micros  =  small. 


UNICELLULAR  ORGANISMS  63 

teria,  the  nutritious  portions  of  which  are  absorbed  by  the  body 
and  assimilated  into  new  Paramecium  substance.  The  un- 
digested refuse  portion  is  eventually  discharged  at  the  posterior 
end  of  the  body  on  one  side.  There  is  no  perrrmnent  opening 
here,  but  whenever  material  is  to  be  rejected  a  temporary 
opening  appears,  at  the  point  shown  at  Fig.  21  ex,  and  the  refuse 
material  is  discharged  into  the  water.  The  process  by  which 
food  is  used,  including  the  absorption  of  oxygen  and  the  excre- 
tion of  waste  products,  as  well  as  the  oxidation  of  the  food 
itself,  is  essentially  identical  with  that  in  the  Amoeba.  As  the 
result  of  the  process,  the  food  material  is  eventually  assimilated 
into  new  Paramecium  substance,  and  the  animal  grows,  increas- 
ing in  size  until  it  is  ready  for  reproduction. 

Irritability. — Paramecium  is  totally  lacking  in  sensory  organs 
or  in  a  nervous  system,  but  like  the  Amceba  it  reacts  to  a  variety 
of  stimuli.  If  an  injurious  stimulus  is  applied  to  one  side  of  it, 
the  animal  will  reverse  its  cilia  and  move  away  from  the  irritat- 
ing stimulus.  It  may  move  backward  or  it  may  turn  its  forward 
end  in  any  direction  and  move  off  to  one  side.  It  is  attracted 
by  certain  chemical  stimuli  and  repelled  by  others.  It  is 
affected  by  heat  in  the  same  way  as  the  Amoeba.  It  is  slightly 
affected  by  an  electric  current,  but  is  not  affected  by  ordinary 
light,  although  the  so-called  ultra-violet  rays  have  an  influence 
upon  it.  These  various  reactions  give  to  Paramecia  an  appear- 
ance of  conscious  sensation,  and  it  appears  as  if  they  had  the 
power  of  volition  to  enable  them  to  avoid  irritating  or  unpleasant 
conditions.  But  the  facts  do  not  necessarily  prove  this,  for  it 
is  possible  that  these  reactions  are  ^only  mechanical  responses 
to  stimuli,  such  as  might  be  found  in  other  machinery.  The 
responses,  however,  are  so  complicated,  and  so  resemble  those 
of  truly  conscious  animals,  that  it  leads  one  to  suspect  that  they 
are  actually  conscious  functions. 

Reproduction. — The  ordinary  method  of  reproduction  of  the 
Paramecium  is  by  division  (fission)  similar  to  that  of  Amoeba, 
although  it  is  more  complicated,  since  the  animal  is  more  com- 


64 


BIOLOGY 


.m 


plex  in  structure.  The  first  step  in  the  process  is  the  elongation 
and  division  of  the  micronucleus  into  two  parts,  one  of  which 
comes  to  lie  at  each  end  of  the  animal; 
Fig.  23.  This  is  followed  by  a  similar 
elongation  and  division  of  the  macronu- 
cleus.  The  oesophagus  produces  a  little 
bud  which  develops  into  a  new  oesopha- 
gus, and  then  this  and  the  old  one  move 
apart,  so  that  the  latter  advances  to  the 
front  part  of  the  body,  and  the  former 
lies  in  the  posterior  part.  A  new  mem- 
branella  develops  in  the  oesophagus.  Two 
new  contractile  vacuoles  make  their  ap- 
pearance, one  just  in  front  of,  and 'one 
just  behind,  the  middle  line  of  the  body. 
Meantime  a  constriction  has  been  mak- 
ing its  appearance,  which  gradually  deep- 
ens, cutting  the  animal  into  two  parts 
by  a  cross  division.  The  two  halves 
thus  produced  separate  from  each  other 
and  swim  away  to  live  an  independent 
life.  It  should  be  noted  that  in  this  reproduction  each  of  the 
important  parts  of  the  animal  divides,  so  that  each  of  the  two 
new  individuals  has  a  part  of  each  organ  which  the  original  Para- 
medum  possessed.  This  multiplication  by  division  may  go  on 
almost  indefinitely  if  the  animal  is  properly  fed  and  placed 
under  favorable  conditions.  Ordinarily  it  will  occur  about 
once  in  twenty-four  hours,  although  the  frequency  may  vary, 
becoming  greater  or  less  with  varying  conditions  of  food  and 
temperature.  A  continuous  reproduction  of  this  kind  has  been 
followed  for  over  2500  successive  divisions.  Whether  it  can  go 
on  indefinitely  if  the  conditions  were  favorable  is  not  known. 
It  is  known,  however,  that  under  ordinary  conditions  this  power 
of  reproduction  gradually  becomes  less  and  less,  and  finally 
tends  to  disappear  altogether.  It  is  believed  that  in  nature 


FIG.    23.  —  PARAME- 
C1UM  IN  PROCESS  OF 

DIVISION 

m,  mouth;  mac,    macro- 
nucleus;  mic,  micronucleus. 


UNICELLULAR  ORGANISMS  65 

this  disappearance  of  the  power  of  multiplication  and  the  nat- 
ural disappearance  of  the  race  is  prevented  by  the  occurrence 
of  another  process  known  as  conjugation  (Lat.  con  =  together 
+  jugare  =  to  join). 

Conjugation. — Two  individual  Paramecia  come  together  and 
place  themselves  side  by  side,  adhering  to  each  other  as  shown  in 
Fig.  24  a.  They  do  not  actually  fuse  together,  but  remain  at- 
tached. The  micronucleus  in  each  undergoes  a  series  of  changes 
which  results  in  its  dividing  into  several  parts,  three  of  which 
degenerate  and  disappear;  c.  Soon  the  fourth  divides  again  into 
two,  one  of  which  is  slightly  larger  than  the  other;  d.  The 
smaller  part  resulting  from  this  last  division  passes  over  into 
the  other  of  the  two  conjugating  individuals,  the  two  animals 
thus  exchanging  nuclear  matter  with  each  other,  as  shown  by 
the  arrows  in  d.  This  small  piece  of  the  micronucleus,  thus 
exchanged  by  each  individual,  unites  in  each  case  with  the  larger 
piece  of  the  nucleus  remaining  in  the  other  individual,  and  the 
two  combine  to  form  a  new  nucleus,  a  fusion  nucleus,  shown 
at  /.  The  animals  now  separate,  each  of  them  carrying  off  in 
itself  a  bit  of  the  micronucleus  from  the  other  individual.  The 
old  macronucleus  next  disintegrates  and  disappears  (f),  and  the 
fusion  nucleus  divides  into  eight  parts  (g) ,  three  of  which  soon 
degenerate.  One  of  the  five  that  are  left  remains  as  a  micro- 
nucleus,  while  the  other  four  become  macronuclei,  at  h.  At  this 
stage  of  the  process  each  Paramedum  has  one  micronucleus  and 
four  macronuclei.  Next  the  micronucleus  divides  into  two, 
and  the  entire  animal  divides  at  once  into  two  separate  parts, 
giving  one-half  of  the  micronucleus  to  each  part.  This  gives  two 
individuals,  each  with  a  micronucleus  and  two  macronuclei ;  i  to 
k.  The  process  is  again  repeated,  the  micronucleus  and  the 
whole  animal,  except  the  macronuclei,  dividing;  the  result  is  two 
more  individuals,  each  containing  one  micronucleus  and  one 
macronucleus;  I  to  m.  This  brings  the  animal  back  to  its  original 
condition,  and  now  the  ordinary  process  of  fission  begins  and 
may  go  on  again  indefinitely,  both  micro-  and  macro-nuclei 


66 


BIOLOGY 


FIG.  24. — CONJUGATION  OF  PARAMECIUM 

ption  of  the  various  stages,  see  text;  m 
cleus;  n,  the  micronucleus.     (Modified  from  Maupas.) 


For  the  description  9f  the  various  stages,  see  text;  m,  in  all  cases  represents  the  macronu- 
"~ed  fron 


UNICELLULAR  ORGANISMS  67 

dividing  with  each  subsequent  cell  division.  Apparently  the 
purpose  of  this  conjugation  is  an  interchange  of  the  material 
present  in  the  micronucleus;  for  it  will  be  seen  that  after  con- 
jugation each  of  the  resulting  animals  contains  nuclear  material 
derived  from  the  micronucleus  of  the  other  individual  as  well 
as  from  its  own 

The  Life  Cycle  of  Paramecium.— We  usually  think  of  the  life 
history  of  higher  animals  as  marked  off  in  definite  life  cycles. 
For  example,  from  the  egg  of  the  hen  develops  the  chick,  which 
grows  into  an  adult  hen  and  produces  another  egg  and  thus 
starts  the  process  over  again.  Such  a  life  cycle  we  speak  of  as 
comprising  a  single  generation,  and  by  the  term  individual  we 
refer  to  all  the  stages  of  the  life  of  the  organism  between  one 
point  in  the  cycle  and  the  next  similar  point.  When  we  attempt 
to  think  of  the  Paramecium  in  a  similar  way,  we  find  the  case 
so  modified  that  the  terms  are  somewhat  difficult  to  apply. 
But  still  in  the  Paramecium  we  can  recognize  a  life  cycle  some- 
what similar  to  that  of  other  organisms.  We  shall  learn  in  a 
later  chapter  that  the  life  of  an  animal  like  a  hen  begins  with 
a  single  cell,  which,  dividing  by  a  process  similar  to  that  we  have 
just  studied  in  the  Paramecium,  gives  rise  to  a  large  number  of 
cells;  see  Fig.  15.  These,  however,  remain  attached  to  form 
the  individual  which  we  speak  of  as  the  chick,  which  grows  into 
the  hen,  and  which  is  thus  composed  of  large  numbers  of  cells. 
This  individual  continues  a  separate  existence  and  eventually  a 
single  cell  is  separated  from  it  to  form  another  egg  and  to  start 
the  process  over  again,  in  a  new  individual. 

Now  if  we  compare  these  facts  with  those  just  seen  in  the 
Paramecium,  we  shall  find  that  the  life  cycle  of  the  Paramecium 
is  as  follows:  Starting  in  the  cycle  at  the  point  where  two  ani- 
mals separate  after  conjugation,  there  begins  a  series  of  cell 
divisions  which  rapidly  increases  the  number  of  cells.  The  cells 
at  once  separate  from  each  other,  become  perfectly  independent, 
swimming  apart  as  quite  isolated  animals,  In  this  respect  the 
development  of  the  Paramecium  differs  very  markedly  from 


68  BIOLOGY 

that  of  the  higher  animals  where  the  cells  remain  attached.  But 
the  process  of  division  is  the  same  and  may  continue  for  a  long 
time.  Eventually,  however,  as  we  have  already  seen,  this 
power  of  division  by  the  simple  process  of  fission  becomes  ex- 
hausted, and  the  multiplication  tends  to  die  out.  We  can  per- 
haps compare  this  with  the  old  age  of  a  larger  animal,  for  in  old 
age  we  find  division  becoming  less  and  less  vigorous,  until  it 
finally  ceases  altogether  and  the  whole  generation  of  cells  dies. 
Among  the  larger  animals,  to  prevent  the  extermination  of  the 
race,  a  single  cell,  an  egg,  is  set  aside  to  start  the  process  over 
again,  thus  beginning  the  new  cycle.  In  the  case  of  the  Parame- 
tium,  after  the  ordinary  reproduction  has  gone  on  for  a  long  time 
it  becomes  impaired  in  vigor  and  seems  to  be  started  over  again 
by  this  process  of  conjugation.  The  process  of  conjugation, 
therefore,  corresponds  to  reproduction  by  an  egg  in  one  of  the 
larger  animals  or  plants.  Hence  one  life  cycle  of  the  Parame- 
dum lasts  from  one  period  of  conjugation,  through  all  the  nu- 
merous successive  divisions  by  ordinary  fission,  until  again  the 
conjugation  occurs  to  start  a  new  cycle.  One  generation,  then, 
consists  of  all  the  members  that  arise  between  one  conjugation 
and  the  next;  and  inasmuch  as  these  animals  may  multiply 
almost  indefinitely  by  ordinary  division,  it  is  evident  that  one 
generation  of  Parameda  may  consist  of  thousands  of  organisms 
scattered  over  a  wide  territory.  It  is  evident,  therefore,  that 
the  term  individual  in  the  case  of  the  Paramedum  cannot  have 
the  same  significance  that  it  has  with  the  higher  animals,  since 
the  individual  of  one  of  the  higher  animals  would  correspond  to 
a  combination  of  all  of  the  different  Parameda  that  arise  from 
the  division  of  any  single  cell  that  comes  from  a  process  of  con- 
jugation, until  again  it  enters  into  a  process  of  conjugation  with 
another  cell.  Conjugation  thus  starts  a  new  generation  or  a 
new  individual. 

We  do  not  know  how  long  a  time  may  elapse  between  two 
successive  conjugations  in  the  case  of  a  Paramedum,  nor  do  we 
know  the  conditions  which  bring  about  the  process.  We  are 


UNICELLULAR  ORGANISMS  69 

even  ignorant  as  to  its  exact  purpose,  although  it  apparently 
appears  to  be  a  process  necessary  to  reinvigorate  the  race  and 
prevent  it  from  dying  out  under  the  ordinary  conditions  of 
environment.  The  process  is  evidently  closely  associated  with 
sex  reproduction  in  the  higher  animals  and  plants,  which  is  to  be 
taken  up  in  a  later  chapter.  We  may  even  speak  of  the  youth 
and  maturity  of  a  Paramecium;  by  the  term  youth  meaning  the 
period  of  rapid  cell  division  that  follows  conjugation,  and  by 
maturity  and  old  age,  the  period  of  slower  cell  division  that 
appears  later  in  the  life  cycle  of  the  animal.  Possibly  we  may 
say  that  the  animal  eventually  dies  of  old  age,  by  which  we 
would  mean  that  unless  conjugation  occurs  the  process  of  simple 
division  is  brought  to  an  end  by  exhaustion.  Whether  old  age, 
and  therefore  conjugation,  are  necessary  in  the  life  history  of 
Paramecium  is  not  yet  settled.  Experiments  have  seemed  to 
show  that  under  proper  conditions  fission  may  go  on  almost 
indefinitely,  certainly  up  to  2500  cell  divisions,  without  the 
necessity  of  conjugation,  or  without  seeming  to  produce  any 
impairment  in  the  power  of  division.  In  the  normal  life  of  the 
individual  it  appears  that  conjugation  is  required,  however,  by 
some  of  the  conditions  of  life.  Paramedumy  therefore,  has  a 
definite  life  cycle,  although  we  do  not  know  its  possible  length 
or  the  conditions  which  modify  it. 

PLASMODIUM  MALARIA 

As  an  example  of  a  still  more  minute  animal,  we  will  study  the 
malarial  organism,  Plasmodium  malarice,  which  lives  in  the  hu- 
man body.  Human  blood  contains  minute  circular  disks  known 
as  red  blood  corpuscles  (see  page  192),  within  which  the  malarial 
organisms  may  be  found  in  persons  who  are  suffering  from 
malaria,  or  chills  and  fever.  The  organism  first  appears  as  an 
extremely  minute  body  (Fig.  25  a),  in  shape  somewhat  like 
the  Amoeba,  though  much  smaller.  It  increases  in  size  as 
shown  by  the  successive  figures  a  to  e.  After  reaching  a  size 
which  nearly  fills  up  the  red  blood  corpuscles,  it  breaks  up 


70 


BIOLOGY 


^T^Afc*      £F     f 


FIG.  25.— THE  LIFE  HISTORY  OF  THE  MALARIAL  ORGANISM 

This  is  shown  in  two  cycles,  the  upper  one  taking  place  in  the  human 
red  blood  corpuscles,  and  the  lower  one  in  the  mosquito.  For  description 
ol  the  individual  stages,  see  text.  (From  various  authors.) 


UNICELLULAR  ORGANISMS  71 

into  twelve  to  sixteen  small  spores,  as  is  shown;  /  to  g.  The 
blood  corpuscle  now  breaks  to  pieces  and  the  spores  are  liberated 
into  the  liquid  blood  h.  Each  may  then  make  its  way  into  a 
new  corpuscle  and  repeat  again  the  history  as  already  described. 

Although  this  animal  in  its  general  structure  and  shape  is 
much  like  the  Amoeba,  its  habits  are  totally  different.  While 
growing  in  the  red  blood  corpuscles  of  the  human  body,  it  pro- 
duces the  disease  which  is  known  as  malaria,  chills  and  fever, 
or  fever  and  ague.  The  period  when  the  chills  occur  corresponds 
to  the  time  when  the  blood  corpuscles  have  broken  up  and  the 
spores  are  liberated  into  the  blood.  The  organism  may  continue 
to  repeat  the  above  history  time  after  time  in  the  blood  of  the 
same  person,  the  spores  after  being  liberated  entering  into  new 
corpuscles,  and  again  repeating  their  life  cycle  almost  indefinitely 
and  prolonging  the  disease.  There  are  three  different  species  of 
the  malarial  organisms,  distinguished  by  the  different  length  of 
time  required  for  their  life  cycles.  The  most  common  form  takes 
48  hours,  a  second  species  takes  72  hours,  and  a  third  is  irregular. 

By  the  method  of  reproduction  above  described,  this  organism 
may  multiply  inside  the  blood  of  one  person  but  is  unable  to 
pass  to  a  second  individual.  Malaria  is  therefore  not  communi- 
cable as  long  as  this  process  alone  is  repeated.  But  after  a  time, 
for  some  unknown  reason,  the  organisms  in  the  corpuscles  assume 
two  different  forms  shown  in  Figure  25  at  g  to  i.  One  of  them 
grows  into  a  large  rounded  mass,  while  the  other  develops  sev- 
eral long  motile,  thread-like  bodies,  which  become  detached.  No 
further  change  occurs  unless  the  patient  is  now  bitten  by  a  cer- 
tain kind  of  mosquito  (Anopheles).  If  the  blood  of  a  patient 
is  swallowed  by  this  mosquito,  the  malarial  organisms  undergo 
a  new  series  of  changes.  The  thread-like  bodies  become  de- 
tached from  the  mass  that  produces  them,  and  one  of  them  unites 
with  one  of  the  larger  rounded  masses,  j  and  k.  -This  union  is 
regarded  as  a  sex  union  (see  Chapter  XII),  the  larger  rounded 
mass  being  the  female  cell  (or  egg)  and  the  thread-like  body  the 
male  cell  (or  sperm)  in  the  sexual  union.  After  the  thread-like 


72  BIOLOGY 

body  penetrates  the  egg,  the  nucleus  it  contains  unites  with  the 
nucleus  of  the  egg,  shown  at  k  and  I.  After  this  union  the  com- 
bined mass  grows  rapidly  in  size,  I  to  o,  and  eventually  breaks 
up  into  an  immense  number  of  minute  spores,  p,  greatly  in 
excess  of  those  found  at  the  stage  g  in  human  blood.  These 
minute  spores  lodge  in  the  salivary  glands  of  the  mosquito,  and 
are  ejected  into  the  blood  of  the  person  bitten  by  the  mosquito. 
Thus  a  new  human  individual  is  inoculated  with  the  spores, 
which  find  their  way  into  the  blood  corpuscles  of  the  new  victim 
and  produce  the  disease.  It  is  not  the  most  common  mosquito 
(Culex)  that  is  concerned  in  this  history,  but  one  that  is  ordi- 
narily less  abundant,  a  species  called  Anopheles.  From  these 
facts  it  follows  that  malaria  will  not  occur  in  any  locality  unless 
this  particular  mosquito  is  present;  and  further,  that  only  the 
mosquitoes  which  have  previously  bitten  malarial  patients  will 
be  able  to  carry  the  infection. 

It  will  thus  be  seen  that  the  malarial  organism  passes  through 
two  stages  in  its  life  cycle,  reproducing  itself  in  each  by  the 
production  of  spores,  though  the  spores  are  of  two  different 
kinds;  and  that  at  one  stage  there  is  a  union  of  cells  of  unequal 
size,  which  may  probably  be  regarded  as  a  true  sex  union.  All 
stages  of  its  life  are  passed  within  the  bodies  of  other  animals, 
and  it  is  thus  wholly  parasitic.  The  three  different  species  of 
the  malarial  organism  have  similar  life  cycles,  though  differing 
slightly  in  details. 

The  malarial  organism  passes  through  two  stages,  in  its  life 
cycle,  each  in  different  animals.  Such  a  complicated  history, 
in  which  there  is  more  than  one  distinct  stage,  is  known  as  a 
metamorphosis  (Gr.  meta  =  beyond  +  morphe  =  form).  Many 
other  animals  have  a  metamorphosis,  one  of  the  best-known 
examples  being  that  of  the  butterfly,  which  passes  through 
the  well-known  states  of  egg,  caterpillar,  cocoon,  and  butterfly. 
Another  example  is  the  frog;  see  page  286.  A  metamorphosis 
is  thus  found  both  among  higher  animals  and  also  among  the 
lowest. 


UNICELLULAR  ORGANISMS 


73 


CHILOMONAS 

This  is  an  example  of  a  still  more  minute  organism  found  very 
abundantly  the  world  over  in  water  among  decaying  leaves. 
From  Figure  26  it  will  be  seen  that  its  structure 
is  extremely  simple.  It  has  a  slightly  elongated 
oval  body,  with  a  little  depression  at  one  end, 
at  the  bottom  of  which  food  is  taken  into  the 
animal,  the  depression  serving  as  a  mouth.  There 
are  no  internal  indications  of  organs,  except  a 
small  nucleus.  At  one  end  are  two  filaments 
called  flagella  (Lat.  flagellum  =  a  whip),  which 
have  the  power  of  lashing  to  and  fro.  By  means 
of  their  lashing  the  Chilomonas  is  driven  through 
the  water.  Chilomonas  multiples  by  simply  dividing  FIG.  '  26.  — 

into  two,  essentially  in  the  CHILOMONAS 

A   very   mi- 

same  manner  as  Amceba.  nute,  flagellate, 

unicellular   ani- 
mal,   found     in 
PANDORINA  stagnant  water. 

Pandorina  is  an  animal  very  similar 
in  its  general  structure  to  Chilomonas, 
except  that  it  is  made  up  of  a  number 
of  cells  grouped  together,  instead  of 
a  single  individual  body;  Fig.  28 A. 
The  method  by  which  this  group  is 
formed  is  simple.  The  animal  starts 
as  a  single  cell,  which  divides,  but 

IVJ.      *-'  I     . -1.     VY   V/    kJJLJ.-*  VJTJJAJ      *      i.imJ-i-.**-'  -  -I  •  •         .  .    i  •  J_  1  /* 

ANIMALS,    RELATED    TO  after  division  the  parts,  instead   of 
CHILOMONAS  separating  at  once,  remain  attached, 

A,Gymnodinium;  B,Ceratium.  i     11  •  /•      •     i 

and  there  arises  a  group  of  sixteen 

cells  attached  together.  They  secrete  a  little  mass  of  jelly 
around  themselves  and  the  flagella  projecting  through  this 
jelly  enable  the  whole  spherical  mass  to  be  rotated  as  a 
unit.  The  individual  members  are  somewhat  independent  of 
one  another,  but  are  attached  so  as  to  form  one  single  unit. 
Such  a  group  is  called  a  colony. 


FlG.  27.— TWO  SINGLE-CELLED 


74 


BIOLOGY 


Multiplication. — Reproduction  of  Pandorina  is  of  two  kinds: 
1.  Each  of  the  cells  of  the  colony  divides  into  sixteen  parts, 

which,  however,  remain 
attached  together,  mak- 
ing a  cluster  of  sixteen 
groups  of  sixteen  cells 
each.  Then  the  whole 
colony  breaks  up,  and 
each  group  of  sixteen  cells 
forms  a  new  colony  liv- 
ing independently  of  the 
others;  Fig.  28 B.  Thus, 
by  simply  dividing,  the 
original  colony  produces 
sixteen  others. 

2.  By  the  second 
method  of  reproduction 
a  conjugation  occurs. 
The  cells  of  a  colony 
break  into  either  sixteen 
or  thirty-two  parts,  and 
then  the  whole  mass 
breaks  to  pieces,  each  cell 
separating,  not  only  from  the  colony  but  from  its  sister  cells. 
Among  the  hundreds  of  cells  thus  formed  some  are  smaller  than 
others;  Fig.  28  C  and  D.  After  swimming  around  for  a  while 
one  of  the  smaller  and  one  of  the  larger  cells  unite  with  each 
other;  Fig.  28  E  and  F.  The  combined  mass  then  secretes  a  red 
shell  or  cyst  about  itself  and  remains  dormant  for  a  time,  show- 
ing no  signs  of  motility,  H.  Later,  however,  it  resumes  its 
activity  and  may  divide  into  two  or  three  parts,  which  then 
escape  from  the  cyst  and  swim  around  for  a  time  as  single  cells, 
called  swarm  spores,  /.  Eventually  each  divides  into  sixteen 
cells  which  remain  together,  forming  a  new  colony  like  the, 
original,  J 


H 


FIG.  28. —  PANDORINA,  A  COMMON  FRESH- 
WATER,  COLONIAL,   UNICELLULAR  ANIMAL 

A,  the  animal  in  its  adult  condition. 

B,  showing  the  method  of  reproduction  by  simple 
division,  each  cell   dividing  into  sixteen  parts  and 
the  whole  colony  breaking  up  into  sixteen  colonies. 

C  to  J  shows  the  successive  stages  of  reproduction 
accompanied  by  conjugation ;  C,  the  larger  of  the  unit- 
ing cells;  D,  the  smaller  ones;E,  their  conjugation; 
//,  the  dormant  condition  within  the  cyst.  For  de- 
scription, see  text. 


UNICELLULAR  ORGANISMS 


75 


INTERMEDIATE  ORGANISMS 

The  organisms  thus  far  described  are  always  classed  as  ani- 
mals.   We  will  now  study  two  similar  organisms,  which  stand 
midway   between  animals  and   plants. 
They  are  closely  related,  and  yet  one 
of  them  is  not  infrequently  classed  as  a 
plant,  while  the  other  is  almost  always 
placed  with  the  animals. 

PGRANEMA 

Peranema  is  a  microscopic  organism 
found  in  stagnant  fresh  water;  Fig.  29  A. 
It  is  elongated  and  tapers  slightly  in 
front.  At  the  narrower  end,  which  is 
carried  forward  in  locomotion,  there 
projects  a  long  motile  flagellum,  by 
the  motion  of  which  the  animal  is  moved 
through  the  water.  At  the  base  of  this 
flagellum  is  an  opening  in  the  animal, 
constituting  a  mouth,  leading  into  a 
short  cesophagal  tube.  At  the  bottom 
of  this  tube  is  a  peculiar  little  rod-shaped 
organ,  which  apparently  serves  as  a  suck- 
ing organ  for  seizing  food.  Near  by  is 
a  clear  contractile  vacuole.  The  proto- 
plasm of  which  the  body  is  made  is  ex-  FIG.  29.— Two  SINGLE- 
tremely  flexible,  and  the  animal,  instead  CELLED  ORGANISMS  RE- 
of  retaining  its  shape,  shows  a  variety 
of  irregular  wavelike  contractions  pass- 
ing  from  end  to  end.  A  nucleus  is 
present,  and  the  animal  moves  either  mother  respects  they  are  much 
by  the  motion  of  its  flagella  or  by 

creeping  somewhat  after  the  fashion  of  the  Amoeba.      As    it 
possesses  a  mouth  and  an  resophagal  tube,  it  lives  on  solid 


SEMBLINGBOTH  ANIMALS 

AND   PLANTS 

A,     Peranema;    B, 


76  BIOLOGY 

food  and  thus  resembles  Paramecium  and  the  other  animals 
already  described. 

EUQLENA 

Euglena  (Fig.  29  B)  greatly  resembles  Peranema  in  shape  and 
structure.  Like  the  Peranema,  it  has  an  elongated  body,  taper- 
ing, however,  at  both  ends.  One  end  carries  a  long,  motile  flagel- 
lum  by  means  of  which  the  animal  moves  through  the  water.  It 
is  made  up  of  flexible  protoplasm  and  goes  through  a  series  of 
contorted  motions  similar  to  those  seen  in  Peranema.  One  or 
more  contractile  vacuoles  are  found  near  the  base  of  the  flagel- 
lum.  The  animal  moves  about  either  by  its  flagellum  or  by  the 
creeping  motion  noticed  in  Peranema.  It  has  also  a  reddish 
"eye  spot"  near  the  front  end. 

Evidently,  these  two  organisms  are  very  closely  related.  In 
two  respects,  however,  there  is  a  striking  difference,  which  has 
led  to  the  classification  of  the  Euglena  by  some  biologists  among 
the  plants  instead  of  among  the  animals.  The  Euglena  probably 
possesses  no  true  mouth  and  does  not  take  in  solid  food,  though 
this  is  disputed.  Moreover,  this  animal  is  green,  and  since 
green  coloring  matter  is  one  of  the  distinctive  characters  of 
plants,  its  presence  in  Euglena  has  led  to  much  controversy 
regarding  the  classification  of  this  organism.  Peranema  with 
its  mouth  and  the  animal  habits  should  evidently  be  classed 
with  the  animals,  whereas  Euglena,  with  its  green  color,  would 
naturally  be  classed  with  the  plants;  and  yet  their  similarity 
would  lead  to  classing  them  together.  A  further  consideration 
of  this  subject  will  be  given  in  a  later  chapter. 

PLANTS 

Although  there  is  a  difference  of  opinion  in  regard  to  the 
classification  of  Euglena  and  Peranema,  there  is  none  in  regard  to 
the  organisms  which  are  now  to  be  described.  The  following 
organisms  are  always  recognized  as  plants,  although  some  of 
them,  for  reasons  that  will  be  given  later,  have  certain  charac- 


UNICELLULAR  ORGANISMS 


77 


ters  that  have  caused  biologists,  in  the  past,  to  group  them  with 
animals.  Modern  scientists,  however,  are  unanimous  in  opinion, 
grouping  the  following  organisms  among  the  plants. 

PLEUROCOCCUS 

Pleurococcus  appears  like  a  green  stain,  growing  in  abundance 
upon  damp  tree  trunks,  fence  posts,  or  even  damp  rocks.  Upon 
scraping  off  some  of  the  material 
and  examining  it  with  a  microscope 
it  is  found  to  consist  of  a  great  num- 
ber of  small  green  cells.  These 
(Fig.  30)  are  spherical,  and  contain 
no  visible  internal  organs  except  a 
nucleus.  The  cells  are  found  massed 
together  into  irregular  bunches, 
but  are  not  really  attached  together. 
As  they  grow  in  size  they  divide  by 
fission  in  two  parts,  each  of  which 
divides  subsequently,  the  new  in- 
dividuals sometimes  remaining  at- 
tached, to  form  irregular  masses 
which  are  easily  shaken  apart.  No 
other  method  of  reproduction  is 
known.  It  is  possible  that  this  little 
plant  is  really  a  stage  in  the  life  of 
some  higher  plant  whose  develop- 
ment is  not  yet  known,  since  it 

has  been  shown  that  some  of  the  more  complex  plants  have  a 
stage  in  which  they  are  simple  green  cells  like  Pleurococcus. 
Concerning  this  organism,  however,  nothing  is  known  positively 
except  that  it  occurs  abundantly  in  damp  places  and,  so  far 
as  known,  has  no  other  phase  of  its  life  than  that  already 
noticed. 


FIG.  30. —  PLEUROCOCCUS 


a,  a  single  cell;  6,  one  showing 
division  by  fission;  c,  a  later  stage 
of  division.  The  plant  in  its  grow- 
ing condition  is  bright  green. 


78 


BIOLOGY 


SACCffAROM  YCES— YEAST 

The  yeast  is  a  plant  slightly  smaller  than  Pleurococcus  but 
resembling  it  in  its  general  shape,  although  it  differs  in  some 

important  respects.  It  is  made 
up  of  single  cells,  usually  slightly 
oval  in  shape,  although  some- 
times they  are  elongated  and 
occasionally  spherical;  see  Fig. 
31.  These  organisms  are  ex- 
tremely minute  in  size,  not 
being  more  than  1/4000  of  an 

FIG.  31.- YEAST  CELLS  inch  in  diameter.     They  are  so 

showing  budding  and  formation  of  groups  small   that  almost  no  internal 

structure  can  be  seen,  although 

each  one  of  them  possesses  a  nucleus  and  a  small  vacuole 

which    is    not    contractile; 

Fig.  32.     As  each  of  these 

bodies  possesses  a  nucleus, 

it  is  a  cell,  and  thus  we  see 

that  the  yeast  is  made  up 

of  clusters  of  single  cells. 
Reproduction. —  The 

method  of  reproduction  of 

yeast  is  by  the   growth  of 

buds  on  the  side  of  the  old 

cell.    The  bud  appears  first 

as  a  swelling,  which  grows 

until  it  is  the   size  of  the  FIQ    32>_YEAST  CELLS  MORE  HIGHLT 

original  cell,  and  may  then      MAGNIFIED     AND     WITH      INTERNAL 

break  away  and  become  an      STRUCTURE  SHOWN 

independent  cell  (Fig.  32),       n,  the  nucleus; 

v,  the  vacuole; 

Of  Several   Of  them  may   re-  *»  shows  spores  in  the  spore  sac  or  ascus. 

_    •          11       u    J   j.         4-1^.^     t  The  figures  show  that  in  budding  the  nucleus 

mam  attached  together  tor  divide8j  Kone  portioil  of  it  pa8Sing*into  the  bud 

some  time,  forming  a  group  and  the  other 

of  more  or  less  independent  cells.  This  process  is  called  budding. 


UNICELLULAR  ORGANISMS  79 

A  second  type  of  reproduction  sometimes  occurs  in  some 
species  of  yeast.  Under  conditions  not  yet  clearly  understood, 
the  contents  of  a  yeast  cell  breaks  up  into  two,  three,  or  four 
parts  which  become  surrounded  by  thick  walls;  Fig.  32  s.  These 
are  called  spores,  or  ascospores,  because  held  in  an  ascus  (Gr. 
ascus  =  sac)  or  sac,  and  eventually  they  are  liberated  by  the 
breaking  of  the  sac.  Each  spore  is  tnen  capable  of  starting  a 
new  series  of  generations  of  ordinary  yeast  cells.  The  spores 
can  resist  drying  and  therefore  serve  to  protect  the  yeast  from 
adverse  conditions. 

A  comparison  of  Figures  30  and  31  will  show  that  yeast  and 
Pleurococcus  greatly  resemble  each  other  in  structure;  but  there 
is  one  important  difference  between  them,  for  Pleurococcus  is 
green  and  yeast  is  colorless.  This  difference  in  color  makes 
a  very  great  difference  in  their  life;  see  page  131.  Whereas 
Pleurococcus  may  grow  luxuriantly  upon  a  fence  post,  and  even 
bare  rocks,  feeding  upon  the  gases  of  the  air,  yeast  is  unable  to 
live  and  grow  unless  it  is  fed  upon  some  organic  matter,  like 
sugar.  While  yeast  cells  may  be  found  widely  distributed  in  the 
air,  in  the  soil,  and  in  the  water,  they  grow  only  where  they  find 
organic  food  to  eat,  and  chiefly  in  solutions  containing  sugar,  like 
fruit  juices,  etc.  Elsewhere,  in  the  soil  or  air,  while  they  may  be 
alive,  they  are  dormant. 

The  chief  function  of  yeast  in  nature  is  to  convert  sugars  into 
carbon  dioxid  and  alcohol.  Sugar  is  produced  in  great  quanti- 
ties by  various  fruits  and  vegetables,  and  is  eventually  attacked 
by  the  numerous  yeasts  that  are  floating  in  the  air.  After  the 
yeasts  have  acted  upon  it,  the  sugar  disappears  and  in  its  place 
can  be  found  a  gas,  carbon  dioxid  (CO2),  and  a  liquid,  alcohol 
(C2H6O).  This  is  called  fermentation,  and  it  is  used  extensively 
in  the  fermentative  industries  which  produce  alcoholic  beverages, 
like  beers,  wines,  ales,  brandies,  etc.  The  fermentation  by  yeasts 
is  also  made  use  of  in  the  raising  of  bread.  The  yeast  growing 
in  the  midst  of  bread  dough  produces  bubbles  of  carbonic  acid 
gas  which  cause  the  solid  heavy  dough  to  become  light  and 


80 


BIOLOGY 


spongy.  The  bread  made  from  such  dough  is  full  of  holes,  and 
is  more  palatable  and  digestible  than  bread  cooked  from  dough 
that  has  not  been  rendered  light  and  porous  (i.e.,  unleavened 
bread).  In  the  case  of  bread  raising  and  beer  making,  the  yeast 
as  a  rule  is  intentionally  planted  in  the  material  which  is  to  be 
fermented.  In  the  making  of  wines  or  the  making  of  cider, 
yeast  is  not  planted.  In  these  cases,  the  grape  juice  or  the  apple 
juice  is  allowed  to  stand  undisturbed,  and  the  yeasts  that  are 
floating  around  in  the  air,  known  sometimes  as  "wild  yeasts," 
have  an  opportunity  of  getting"  into  the  juices,  where  they  grow 
and  produce  fermentation.  Thus  although  no  yeast  has  been 
added  to  these  materials,  the  fermentation  is  brought  about  by 
yeast  exactly  as  if  the  yeast  had  intentionally  been  added. 

BACTERIA 

The  simplest  of  all  known  living  organisms  are  the  Bacteria. 
These  consist  of  the  extremely  minute  organisms  shown  in  Figure 

33.  Some  of  them  are  spher- 
ical, some  are  in  the  form  of 
short  rods  or  long  threads, 
and  some  spiral.  They  are  so 
minute  that  practically  no 


o  oo  cooo 


FIG.  33.— BACTERIA 

A,  rod-shaped  form,  Bacillus  or  Bacte- 
rium; 1,  Diphtheria  bacillus;  B,  spiral  forms, 
Spirillum;  C,  spherical  forms,  Coccus;  2, 
Streptococcus;  D,  the  method  of  multiplica- 
tion by  division ;  E,  the  formation  of  spores,  s. 


FlG.    34. A  DIAGRAM  SHOWING  THE 

RELATIVE    SIZE    OF   THE    POINT   OF 
A   FINE   NEEDLE   AND   BACTERIA 

The  small  dots  at  the  tip  of  the  needle 
represent  bacteria. 


internal  structure  can  be  seen.    Some  of  them  are  not  more 
than   1/50,000   of   an  inch  in  diameter;  see  Fig.  34.     Some 


UNICELLULAR  ORGANISMS 


81 


FIG.  35. —  BACTERIA  WITH  FLAGELLA 

A,  flagella  are  distributed  over  the  whole 
body,  a  condition  called  peritrichic;  B,  flagella 
grouped  together  in  cluster  at  one  end,  called 
Jophotrichic;  C,  a  single  flagellum,  monolrichic* 


bacteria  (see  Fig.  35)  have  minute  flagella,  which  by  lashing  to 
and  fro  cause  them  to  move.  Beyond  the  points  shown  in  the 
figures,  there  is  very  little  to 
be  said  concerning  the  struc- 
ture of  bacteria. 

Reproduction.  —  Bacteria 
all  multiply,  by  fission,  each 
dividing  into  two  parts, 
which  again  divide  when 
they  have  grown  to  the 
size  of  the  parent  cell. 

Spore  Formation. — Some 
species  of  bacteria  produce 
spores  in  the  following  man- 
ner: After  growing  for  a 
time  by  division  the  contents  of  a  single  bacterium  collect  into 
a  rounded  mass  which  becomes  surrounded  by  a  hard  resisting 
wall;  see  Fig.  33 E.  This  is  set  free  by  the  breaking  of  the  bac- 
terium that  holds  it  and  is  then  capable  of  starting  a  new  series 
of  generations.  This  clearly  resembles  the  ascospore  formation 
in  yeast,  except  that  there  is  no  actual  multiplication  of  indi- 
viduals, one  bacterium  giving  rise  to  one  spore  only.  The 
spores  have  resisting  walls  and  are  able  to  stand  drying  and 
a  fairly  high  degree  of  heat.  Their  function  is  thus  that  of 
protecting  the  race  from  destruction  by  drying  and  heat  rather 
than  that  of  multiplication,  the  latter  function  being  performed 
by  the  process  of  simple  division. 

Bacteria  are  very  widely  distributed  in  nature.  They  are 
found  in  the  air,  in  the  soil,  in  all  bodies  of  water,  and,  in  fact, 
practically  everywhere.  They  play  an  extremely  important 
part  in  the  life  processes  of  nature  through  their  relation  to  all 
forms  of  putrefaction,  decomposition,  and  decay.  The  bacteria 
are  important  agents  in  maintaining  the  continued  fertility 
of  the  soil,  making  it  capable  of  producing  crops  year  after 
year.  A  few  species  live  as  parasites  within  human  bodies 


*Gr.  peri  =  around 
Gr.  lophos  =  tuft 
Gr.  monus  =  one 


trix  =  hair. 


&  BIOLOGY 

and  th«ee  of  animals.  These  are  pathogenic  bacteria  or  disease 
germs.  They  cause  many  of  our  most  serious  contagious 
diseases  like  typhoid  fever,  tuberculosis,  diphtheria,  blood  poison- 
ing, etc.  Thus,  although  they  are  extremely  minute,  bacteria 
are  agents  of  great  importance  in  the  world.  It  is  hardly 
possible  to  imagine  anything  more  simple  in  structure,  but 
at  the  same  time  of  greater  importance,  than  bacteria. 


LABORATORY  WORK 

The  best  method  of  obtaining  material  for  laboratory  work  is  to  place 
in  a  number  of  glass  jars  or  shallow  dishes  pond-lily  leaves,  leaves  of  other 
plants,  algae  of  various  kinds,  or  any  other  decaying  organic  material 
from  ponds  and  ditches.  Fill  the  dishes  with  water  and  allow  them  to  stand 
undisturbed  from  one  to  several  weeks.  Various  kinds  of  microscopic 
organisms  will  appear  in  the  different  dishes,  from  which  the  desired  organ- 
ism can  be  chosen. 

Amoeba. — A  brown  scum  will  usually  appear  in  a  few  days  on  the  surface 
of  the  water  covering  the  decaying  organic  material  which  is  likely  to  contain 
Amoebae.  When  this  scum  is  scraped  from  the  leaves  and  studied  under  a 
1/6  inch  objective  it  will  usually  disclose  small  specimens  of  Amoeba.  The 
animals  should  be  studied  alive  and  without  any  special  treatment,  since 
they  are  sufficiently  transparent,  and  slow  enough  in  their  movements  to 
show  all  the  points  in  their  anatomy,  and  nearly  all  the  features  mentioned 
in  the  text  may  be  seen  without  difficulty. 

Paramedum. —  These  may  be  found  in  abundance  in  the  scum  from  the 
decaying  pond  weeds  after  they  have  been  left  for  a  week  or  more.  Many 
white,  moving  bodies,  just  visible  to  the  naked  eye,  will  be  found  in  a  drop 
of  this  scum,  which  should  be  studied  with  a  1/6  inch  objective.  The  chief 
difficulty  in  studying  them  is  due  to  their  constant  motion;  various  methods 
of  holding  them  quiet  may  be  used.  A  bit  of  filter  paper  under  the  cover 
glass  will  sometimes  hold  the  individuals  quiet  in  its  meshes,  or  they  may  be 
held  quiet  under  a  cover  glass  by  supporting  it  on  a  small  bit  of  paper,  just 
thick  enough  to  hold  them  without  crushing  them.  The  animals  are 
to  be  studied  alive,  and  a  little  patient  examination  of  several  specimens 
will  usually  show  most  of  the  points  of  structure  mentioned  in  the  text. 
To  bring  out  the  nucleus,  a  very  weak  aqueous  solution  of  methyl  green 
should  be  run  under  the  cover  glass.  If  the  solution  is  not  too  strong  it  will 
stain  the  nucleii  green,  before  affecting  the  rest  of  the  organism.  Animals 


UNICELLULAR  ORGANISMS  83 

in  the  state  of  division  may  readily  be  found.  Conjugation,  however,  is 
rare  and  cannot  be  studied  by  a  class. 

The  other  unicellular  animals  mentioned  in  Chapter  II  may  be  commonly 
found  with  Amceba  and  Paramedum.  They  cannot  always  be  obtained, 
however,  and  the  student  will  often  be  obliged  to  omit  them.  Euglena 
should  not  be  omitted,  however,  if  any  appear  in  the  dishes  of  decaying 
pond  weeds. 

Pleurococcus. — The  best  method  of  obtaining  this  for  studyis  to  find  some 
fence  post  or  log  which  is  covered  with  a  green  growth.  This  material 
scraped  from  the  wood  will  usually  prove  to  be  a  mass  of  Pleurococci.  No 
special  method  of  study  is  needed  except  to  place  a  small  quantity  in  a 
drop  of  water  and  study  with  a  1/6  inch  objective.  The  structure  can  be 
readily  seen  and  cells  may  be  found  showing  division  by  fission. 

Yeast. — A  cake  of  ordinary  compressed  yeast  furnishes  excellent  material. 
A  small  quantity  should  be  rubbed  with  a  little  water  in  a  watch  glass.  A 
minute  drop  of  this  material  diluted  still  further  in  water,  and  studied  with 
a  1/6  inch,  will  show  the  structure  of  the  yeast  except  the  nucleus,  which 
can  only  be  made  out  by  special  methods.  Many  cells  showing  buds  may 
be  found  in  a  fresh  yeast  cake.  Such  a  yeast  preparation  usually  contains 
grains  of  starch,  which  may  be  distinguished  from  the  yeast  by  running 
a  little  iodine  solution  under  the  cover  glass,  which  will  turn  the  starch  blue. 
The  starch  has  nothing  to  do  with  the  yeast,  being  added  to  the  cake  to 
give  it  body.  A  few  drops  of  the  yeast  emulsion  should  be  planted  in  several 
large  test  tubes  containing  a  fermentable  liquid.  Pasteur's  solution  is  best, 
but  a  little  diluted  molasses  will  serve.  Pasteur's  solution  contains  the 
following  ingredients :  — 

Water 837.60  c.  c. 

Grape  sugar 150  gms. 

Ammonium  tartrate      ..........  10  " 

Potassium  phosphate 2 

Calcium  phosphate .2  " 

Magnesium  sulphate .2  " 

1000 

If  these  tubes  are  placed  in  a  warm  place,  80°  to  90°  F.,  fermentation  will 
soon  begin,  and  after  a  few  hours  bubbles  of  CO2  may  be  seen  rising  through 
the  liquid.  After  12  hours  a  little  of  the  scum  or  the  sediment  will  show  the 
actively  growing  yeast.  This  growing  yeast  should  be  carefully  compared 
with  the  fresh,  dormant  yeast  in  the  yeast  cake. 

Bacteria, — Only  a  little  work  can  be  done  without  special  methods  which 
are  complicated  and  difficult.  Bacteria  may  be  shown,  however,  as  follows: 


84  BIOLOGY 

Spread  a  bit  of  any  decaying  matter  (the  decaying  pond  weeds  will  do  very 
well,  or  a  bit  of  tartar  scraped  from  the  teeth)  in  as  thin  a  film  as  possible 
upon  a  slide,  dry  in  air  or  fix  by  heat  by  passing  it  twice  through  a  gas  flame. 
When  thoroughly  dry  flood  the  slide  with  a  solution  of  fuchsin  or  methylene* 
blue  and  allow  to  stain  for  two  to  five  minutes.  Then  wash  the  stain  off  in 
running  water,  and  place  a  cover  glass  over  the  stained  mass  on  the  slide. 
The  bacteria  appear  under  a  high  power  objective  as  minute  stained  dots, 
or  short  rods.  They  are  much  smaller  than  yeast  cells,  and  are  only  just 
visible  with  a  1/6  inch  objective.  Higher  powers  are  needed  to  study 
them. 

BOOKS  FOR  REFERENCE 

BRONN,  Klassen  und  Ordnung  des  Thierreichs,  C.  F.  Winter,  Leipzig. 

DAVIDSON,  Practical  Zoology,  American  Book  Company,  New  York. 

HEGNER,  Introduction  to  Zoology,  Macmillan  Co.,  New  York. 

HEGNER,  College  Zoology,  Macmillan  Co.,  New  York. 

HERTWIG,  Manual  of  Zoology,  translated  by  Kingsley,  Henry  Holt 
&  Co.,  New  York. 

JORDAN  and  PRICE,  Animal  Structures,  D.  Appleton  Company,  New 
York. 

MARSHALL,  Microbiology,  P.  Blakiston's  Son  &  Co.,  Philadelphia. 

PARK,  Pathogenic  Bacteria  and  Protozoa,  Lea  &  Febiger,  New  York. 

PARKER,  Elementary  Biology,  Macmillan  Co.,  New  York. 

PARKER  and  HASWELL,  Text-book  of  Zoology,  Macmillan  Co.,  New 
York. 

PRATT,  Invertebrate  Zoology,  Ginn  &  Co.,  Boston. 


*Methylene  blue  solution  is  made  as  follows:  — 

Saturated  alcoholic  solution  of  methylene  blue  .      .      .     15  c.  c. 

Potassium  hydrate  (1 : 10,000) 50  c.  c. 

To  make  a  1 : 10,000  solution  of  KOH,  add  1  c.  c.  of  a  10%  solution  ,to 
99  c.  c.  of  water  and  then  add  5  c.  c.  of  this  to  45  c.  c.  of  water. 


CHAPTER  IV 

CELL  MULTIPLICATION  AND  THE  CELLULAR 
STRUCTURE  OF  ORGANISMS 

BEFORE  undertaking  the  study  of  the  multicellular  organisms 
we  must  study  in  detail  the  process  by  which  cells  multiply. 
We  have  already  seen  that  the  Amoeba,  Paramecium,  and  other 
single-celled  animals  and  plants  have  the  power  of  dividing. 
Indeed  all  active,  growing  cells  have  the  power  of  multi- 
plying by  division.  Although  division  seems  a  very  simple 
process,  in  reality  it  is  unexpectedly  complex.  The  internal 
changes  in  the  cell  during  division  have  been  made  out  only 
by  long  study.  While  they  differ  in  many  small  details, 
all  cells  agree  in  certain  broad  general  facts.  The  process 
known  as  karyokinesis  or  mitosis  (Gr.  mitos  =  thread)  is  alike 
in  outline  in  most  cells  and  is  as  follows :  — 

CELL  DIVISION  OR  KARYOKINESIS  * 

The  Resting  Cell. —  In  Figure  36^4.  will  be  seen  a  cell  in 
the  condition  of  rest,  before  it  has  passed  into  the  stage  of 
division.  It  will  be  noticed  that  the  centrosome  is  in  the  form 
of  two  minute  granules,  and  that  the  chromatin  inside  of  the 
nucleus  is  in  the  form  of  a  diffused  network.  No  other  factors 
need  concern  us  at  the  present  time. 

1.  Prophase. — The  first  stage  in  the  division  involves  both 
the  nucleus  and  the  centrosome.  In  the  nucleus  the  chromatin 
assumes  the  form  of  a  long  thread  sometimes  known  as  the 
spireme.  This  condition,  however,  is  only  preliminary  to  the 
breaking  up  of  the  thread  into  a  number  of  short  pieces  which 
are  called  chromosomes  (Gr.  chroma  =  color  +  soma  —  body) ; 
Fig.  36  B.  The  number  of  chromosomes  which  arise  in  the 
nucleus  varies  with  different  organisms  but  is  constant  for  each 
species  of  organism  and  is  always  an  even  number.  In  the 

*As  here  described  karyokinesis  applies  chiefly  to  animal  cells. 
85 


BIOLOGY 


type  represented  in  Figure  36, 
is  invariably  four. 


G 

FIG.  36.  —  DIAGRAM     SHOWING     THE 

SUCCESSIVE   STAGES   IN   THE    PROCESS 
OF  KARYOKINESIS 

A,  the  resting  cell  before  it  enters  into  the 
process  of  cell  division;  H,  the  completed  proc- 
ess after  the  cell  has  divided  into  two  parts; 
ce,  the  centrosome;  ch,  the  chromatin.  For 
description  of  the  different  stages,  see  text. 

spindle,  known  as  the  equatorial 
chromosomes  and  the  separation 


the  number  of  chromosomes 

The  second  part  of  the 
first  stage  consists  of  the 
separation  of  the  two  gran- 
ules of  the  centrosome  as 
shown  at  B.  As  these  parts 
separate,  they  are  seen  to 
be  connected  by  fibers  form- 
ing what  is  called  the  spin- 
dle. The  granules  continue 
to  move  away  from  each 
other  until  they  finally 
come  to  lie  at  opposite 
poles  of  the  nucleus,  form- 
ing the  amphiaster  (Gr.  am- 
phi  =  both  -f  aster  =  star) 
as  shown  at  D.  They  are 
still  connected  by  the  fibers 
of  the  spindle,  which  now 
pass  into  the  nucleus  itself; 
the  nuclear  membrane  in 
the  meantime  has  disap- 
peared. At  the  end  of  this 
phase  the  chromosomes 
have  assumed  a  position 
midway  between  the  two 
granules,  lying  on  the  mid- 
dle of  the  spindle,  and  at 
right  angles  to  the  line  con- 
necting them,  at  E.  They 
thus  form  a  sort  of  plate  be- 
tween the  two  poles  of  the 
plate.  The  formation  of  the 
of  the  centrosomes  may  take 


CELL  MULTIPLICATION  87 

place  simultaneously,  or  one  of  them  may  precede  the  other; 
the  relative  order  of  these  changes  varies  and  is  a  matter  of 
no  especial  importance. 

2.  Metaphase. — The  second  stage  in  cell  division  is  a  very 
important  one  and  is  really  the  key  to  the  process.     Each  of 
the  chromosomes  splits  lengthwise  into  two  identical  halves, 
which  at  first  are  parallel,  as  at  D.    This  splitting  of  the  chro- 
mosome into  identical  halves  is  for  the  purpose  of  dividing 
equally  the  chromatin  material,  so  that  the  two  cells  which 
are  to  arise  from  the  original  cell  may  each  contain  one-half 
of  the  chromatin  rods  of  the  original  cell.     The  fact  that  the 
chromosomes  split  lengthwise  is  of  significance,  for  it  is  mani- 
fest that  if  the  thread  splits  lengthwise,  the  two  halves  will 
be  essentially  identical,   while   if  it  should  divide  crosswise, 
the  two  halves  would  not  be  necessarily  alike.     In  the  equa- 
torial plate,  at  E,  these   eight  chromosomes  become  slightly 
flattened  and  are  drawn  more  closely  together. 

3.  Anaphase. — In  the  third  stage,  the  two  halves  of  each 
chromosome  begin  to  move  apart.     As  shown  at  F,  four  of 
the  chromosomes  move  away  from  the  equatorial  plate  toward 
each  of  the  two  centrosomes.     There  is  little  doubt  that  the 
minute  fibers  which  connect  the  poles  of  this  spindle  are  con- 
cerned in  the  separation  of  these  chromatin  threads,  though 
exactly  how  they  work  is  not  known.     Finally,  the  separate 
halves  of  the  chromatin  thread  are  brought  close  to  the  minute 
granules  lying  at  the  two  ends  of  the  spindle,  at  G. 

4.  Telophase. — The  last  stage  in  the  division  simply  com- 
pletes the  process,  for  the  essential  feature  of  division  has 
already  occurred.  The  chromatin  threads,  which  have  come 
to  lie  near  the  pole  of  the  spindle,  now  combine  and  form  a 
network,  at  G,  much  like  that  present  in  the  original  nucleus, 
and  a  nuclear  membrane  forms  around  this  mass  of  chromatin 
material  at  H.     The  minute  granule  within  the  center  of  the 
spindle  pole  is  divided  in  two,  either  now  or  later;  and  thus 
a  complete  nucleus  is  produced  with  a  centrosome  beside  it, 


88  BIOLOGY 

containing  two  granules,  at  H;  this  nucleus  is  an  exact  repe- 
tition of  the  one  with  which  we  started.  Meantime  a  division 
plane  forms,  passing  through  the  cell  midway  between  these 
reconstructed  nuclei,  and  the  division  of  the  cell  into  two 
parts  is  now  completed.  There  are  thus  produced  two  cells, 
identical  with  each  other  and  identical  with  the  original  cell, 
each  with  similar  chromatin  material,  since  each  contains 
half  of  the  original  chromosomes.  By  this  process,  therefore, 
the  chromatin  of  the  nucleus  is  continuous  from  one  cell  gen- 
eration to  another. 

It  will  be  evident  that  the  essential  purpose  of  this  cell 
division  is  the  splitting  of  the  chromatin  material  into  identical 
halves.  It  would  seem  much  simpler  for  the  cell  to  divide 
immediately  into  two  parts  without  this  long  process;  but  this 
might  not  make  the  two  parts  equivalent.  In  order  that  they 
may  be  equivalent,  the  cell  adopts  the  complicated  process 
of  karyokinesis.  In  the  case  described,  the  two  final  cells 
are  practically  of  equal  size;  but  even  in  instances  where  the 
cells  finally  produced  are  of  very  unequal  size  (Fig.  121), 
the  amount  of  chromatin  in  each  is  the  same.  Since,  therefore, 
the  essential  purpose  of  this  process  of  karyokinesis  is  the 
splitting  of  the  chromatin,  it  is  evident  that  this  material 
must  be  of  extreme  significance  in  the  life  of  the  cell.  When 
we  combine  this  knowledge  with  the  fact  mentioned  in  Chapter 
II,  that  the  cell  can  carry  on  its  life  processes  only  when  it 
has  nuclear  material,  it  becomes  manifest  that  the  nucleus, 
instead  of  being  a  negligible  part  of  the  cell,  is  really  the  cen- 
tral feature  of  its  life. 

Nuclear  Division  without  Cell  Division. — As  a  rule,  almost 
immediately  after  the  nucleus  completes  its  division,  the  body 
of  the  cell  divides  so  that  a  cell  does  not  contain  more  than 
a  single  nucleus  for  any  length  of  time.  Occasionally,  however, 
the  division  of  the  cell  body  is  delayed  and  the  nucleus  divides 
a  second  time,  and  perhaps  several  times,  before  the  cell  body 
divides,  the  result  being  one  mass  of  protoplasm  containing 


CELL  MULTIPLICATION 


89 


several  nuclei.  In  most  instances  the  division  of  the  cell  is 
simply  delayed  and  takes  place  later,  so  that  finally  the  con- 
dition of  a  single  nucleus  in  each  cell  is  resumed.  This  occurs 
in  the  dividing  egg  of  insects,  for  example.  In  some  instances, 
however,  the  cell  body  does  not  divide  at  all,  and  the  continued 
division  of  the  nucleus  produces  a  connected  mass  of  proto- 
plasm with  many  nuclei.  This  occurs,  for  example,  in  some 
molds  shown  in  Figure  42  E,  in  which  there  is  no  sign  of  cell 
division,  although  there  are  many  nuclei.  Such  a  condition 
is  called  a  syncytium  (Gr.  syn  =  together  +  cytos  =  cell)  and 
is  sometimes  described  as  acellular.  This  multicellular  state 
with  incompleted  cell  division  is  rare,  for  in  most  instances 
the  division  is  completed  promptly. 

Amitosis. — While  division  by  karyokinesis  is  the  common 
method  of  cell  division  among  all  organisms,  there  are  some 
instances  where  cells  divide  without 
going  through  these  stages.  This  is 
most  likely  to  occur  in  the  old  age  of 
the  cell  when  its  vitality  begins  to  de- 
cline. In  these  cases,  the  nucleus  di- 
vides directly ;  sometimes  being  simply 
pinched  into  two  parts  (Fig.  37),  some- 
times being  compressed  into  a  middle 
plate  which  divides  into  two  halves 
and  then  separates,  and  sometimes 
forming  two  nuclear  membranes  in- 
side of  the  original  membrane  which 
then  ruptures  and  permits  the  escape 
of  the  new  nuclei.  In  these  cases, 
it  frequently  happens  that,  though 
the  nucleus  divides,  the  cell  body 
does  not  divide,  so  that  'there  re- 
sults a  cell  with  more  than  one  nucleus.  This  process  of  di- 
vision is  called  amitosis  (Gr.  a  =  without  +  Lat.  mitos  =  thread), 
and  it  is  thought  to  indicate  a  decline  in  the  vigor  of  the  cells. 


d 

FIG.  37. —  DIAGRAM  SHOW- 
ING   THE     PROCESS     OP 
NUCLEAR     DIVISION    BY 
AMITOSIS 
(Modified  from  Wheeler.) 


90 


BIOLOGY 


UNICELLULAR  AND  MULTICELLULAR  ORGANISMS 
All  of  the  organisms  thus  far  studied  have  been  made  up 
of  single  cells,  each  cell  being  independent  and  capable  of 
carrying  on  all  life  processes  within  itself,  although  many  of  them 
are  quite  complex,  having  several  organs  and  much  variety; 
see  Fig.  38.  In  contrast  to  these  unicellular  organisms  we 
shall  find  organisms  made  up  of  large  numbers  of  cells  (multi- 
cellular  organisms).  All  of  the  larger  and  higher  animals 
and  plants  in  the  world  are  made  up  of  great  numbers 
of  cells,  each  having  the  same  general  structure  as  the  uni- 
cellular organisms  we  have  already  studied.  These  larger 
organisms  begin  their  life  as  single  cells  and  become  multi- 
cellular  by  the  division  of  their  cells  into  many  parts. 
There  is  no  doubt  that  the  ~  multicellular  organisms  of  the 

world  must  have  been  de- 
rived originally  from  the 
unicellular  organisms. 

Intermediate  Types.— 
While  the  organisms  de- 
scribed in  the  last  chapter 
are  called  unicellular,  there 
are  some  of  them  to  which 
this  term  cannot  be  ap- 
plied with  strict  accuracy. 
Pandorina  (Fig.  28),  for 
example,  consists  of  a 
group  of  sixteen  cells  at- 
tached in  a  spherical, 
gelatinous  mass.  Each  of 
these  masses  of  sixteen 
cells  has  been  derived 
from  a  single  cell  by  divi- 
sion. It  is  a  question 

whether  this  organism  should  be  called  unicellular  or  multi- 
cellular.     It  is  certainly  made  up  of  more  than  one  cell;  but 


FIG.  38. — BURSARIA.  ONE  OF  THE  LARG- 
EST AND  MOST  COMPLICATED  OF  THE 
SINGLE-CELLED  ANIMALS 


/,  food; 
m,  mouth; 


mb,  membranella; 
mac,  macronucleus. 


CELLULAR  STRUCTURE  OF  ORGANISMS  01 

on  the  other  hand  the  cells  are  all  alike,  are  all  capable  of 
carrying  on  the  various  functions  of  life,  and  may  be  more 
or  less  independent  of  each  other. 

Vorticella  and  Carchesium. — Other  examples  of  types  inter- 
mediate between  unicellular  and  multicellular  forms  are  shown 
in  Figures  39  and  40.  The  Vorticella,  shown  at  Figure  39  A, 


FIG.  39. —  Two  SPECIES  OF  UNICELLULAR  ORGANISMS 

Showing  the  formation  of  colonies.  A,  a  single-celled  Vorticella;  B,  the  process  of  divi- 
sion; C,  a  single  cell  of  Carchesium;  D,  a  colony  of  Carchesium,  produced  by  the  incom- 
plete division.  Vorticella  always  separates  after  division,  but  Carchesium  remains  attached 
as  shown  at  D, 


cv,  contractile  vacuole; 
oe,  O3sophagus; 
m,  mouth; 


mac,  macronucleus; 
mic,  micronucleus. 


is  unquestionably  a  single-celled  animal,  bell-shaped  and  pos- 
sessing cilia,  a  mouth,  oesophagus,  vacuole,  and  a  macro- 
and  micronucleus;  the  whole  is  attached  to  a  stalk  containing 
a  muscle  which  enables  it  to  contract.  This  single  cell  divides 
in  a  normal  manner  (B)  and  after  division  the  parts  separate 


BIOLOGY 


to  become  independent  animals.  In  Figure  C  is  shown  another 
cell  much  like  Vorticella,  possessing  the  same  shape  and  similar 
organs.  In  this  animal,  after  the  cells  divide,  they  do  not 
separate  but  remain  attached  to  a  common  stalk,  and  subse- 
quently divide  again  and  again,  the  result  being  a  group  of 
similar  cells  connected  by  a  branching  stalk,  D.  This  animal 
is  named  Carchesium,  and  such  a  cluster  is  called  a  colony. 
In  this  colony  the  members  are  independent,  each  carrying 
on  for  itself  all  of  the  functions  of  life  and  each  contracting 
and  expanding  by  itself  independently  of  the  rest.  A  third 
species  is  found  resembling  Carchesium  except  in  one  respect. 

In    this    animal,    Zoo- 

\  \ ' '  i •' '•/;///  /  v  thamniwn,    there    is    a 

1 ' '  'i/ &&•••/  /    ^^ 

common  muscle  ex- 
tending through  the 
stalk  and  its  branches. 
When  this  muscle  con- 
tracts, all  the  members 
of  the  colony  contract 
simultaneously. 

These  three  animals 
are  evidently  closely  re- 
lated; but  Vorticella  is  SL 
true  unicellular  animal, 
Carchesium  a  cluster  of 
independent  cells  at- 
tached together,  and 

Zoothamnium  a  similar  colony  in  which  the  members  are  not 
wholly  independent  but  have  a  vital  connection. 

There  are  many  other  animals  which  are  in  a  similar  way 
made  up  of  colonies  of  cells,  alike  in  structure  and  function. 
Several  of  these  are  sho\/n  in  Figure  40.  In  all  cases  the  ani- 
mals start  their  life  as  single  cells  which  become  colonies  by 
the  method  of  incomplete  division.  All  these  are  commonly 
classed  among  unicellular  animals  and  called  Protozoa  (Gr. 


FIG.  40. —  COLONIES  OF  UNICELLULAR  OR- 
GANISMS MADE  UP  OF  SEVERAL  CELLS 
ATTACHED  TOGETHER 


A,  an  animal  with  its  pseudopodia  protruding;  in 
the  other  specimens  only  the  shell  is  visible.  These  ani- 
mals belong  to  the  group  of  Forminifera,  whose  shells 
form  chalk  cliffs  and  limestone  rocks. 


CELLULAR  STRUCTURE  OF  ORGANISMS 


93 


protos  =  first  +  zoon  =  animal),  although  they  are  not  strictly 
unicellular. 

The  same  principle  is  illustrated  by  many  of  the  lower  plants, 
of  which  a  single  example  will  be  given. 

Ulothrix. — One  of  the  common  fresh-water  pond  scums, 
found  everywhere  in  ditches  by  the  roadside,  is  made  of  a 
green  plant,  Ulothrix;  Fig.  41.  Ulothrix  consists  of  a  long, 
slender  thread  formed  by  a  row  of  nearly  cylindrical  cells, 
placed  end  to  end;  Fig.  41  A.  The  individual  threads  are 
barely  visible  to  the  naked  eye.  In  each  one  of  these  cells 
rnay  be  seen  green  coloring  matter,  chlorophyll  (Gr.  chloros  = 
green  +  phyllon  =  leaf),  and  each  cell  contains  a  nucleus. 
The  cells  are  identical  from  one  end  of  the  thread  to  the  other, 
differing  only  slightly  in 
size,  and  each  of  them  is 
capable  of  carrying  on 
all  the  functions  of  life 
independently. 

The  reproduction 
in  Ulothrix  is  interest- 
ing; and,  like  some  or- 
ganisms already  studied, 
it  shows  two  quite 
distinct  methods.  The 
first  and  simplest  is  as 
follows:  The  contents  of 
one  of  the  cells  breaks  up 
into  several  parts,  which, 
after  a  time,  escape  upon 
the  bursting  of  the 
plant's  cell  wall.  As  they 
come  out,  each  is  seen  to 
be  provided  with  four  little  flagella  and  is  thus  enabled  to  swim. 
They  are  called  zoospores  (Gr.  zoon  =  animal) ;  Fig.  41  a.  After 
swimming  for  a  time  they  settle  down,  lose  their  cilia,  and 


FIG.  41.— PLANTS  MADE  UP  OF  COLONIES 
OF  SINGLE  CELLS 

A,  Ulothrix.  a,  shows  the  process  of  multiplica- 
tion by  the  formation  of  zoospores;  b  to  /,  show  the 
formation  of  sex  cells,  their  conjugation  with  each 
other;  g,  their  subsequent  division  into  spores; 
h,  a  single  spore  which  grows  into  a  new  thread,  like 
the  original  shown  at  large  A.  B,  Pediastrium. 


94  BIOLOGY 

each  begins  to  develop  into  a  new  filament  like  that  from 
which  it  originated.  The  growth  into  the  new  filament  is  by 
division;  the  cells  after  dividing  remain  attached  together  in 
the  form  of  a  long  chain. 

The  second  method  of  reproduction  is  by  conjugation  and 
reminds  us  of  that  in  Pandorina.  In  this  case,  the  contents 
of  some  of  the  cells  break  up  into  a  large  number  of  parts 
instead  of  a  small  number,  and  these,  by  the  bursting  of  the 
cell  wall,  are  finally  liberated  into  the  water ;  Fig.  41  c. 
They  are  then  found  to  possess  two  flagella,  instead  of  four 
like  the  zoospores,  and  by  means  of  these  they  swim  around. 
These  small  spores  are,  however,  unable  to  grow  into  new 
threads.*  After  the  spores  have  been  swimming  about  foi 
some  time  they  come  in  contact,  as  shown  in  Figure  41  d,  and 
fuse  together,  the  fusion  being  identical  with  that  already 
described  in  Pandorina;  see  page  74.  There  are  thus  formed 
conjugation  spores  known  as  the  zygospores  (Gr.  zygon  =  yoke). 
These  zygospores,  after  a  time,  produce  by  division  several 
more  spores  which,  upon  becoming  free,  soon  begin  to  divide 
and  grow  into  new  filaments  like  those  with  which  we  started .. 
This  kind  of  reproduction  is  very  similar  to  that  of  Pandorina 
and  clearly  suggests  the  sexual  reproduction  which  occurs  in 
higher  organisms. 

In  the  organisms  thus  described,  we  have  examples  which 
cannot  properly  be  called  unicellular,  nor  on  the  other  hand 
can  they  be  called  multicellular;  each  one  of  these  cells  carries 
on  by  itself  all  the  functions  of  the  organism,  whereas  in  multi- 
cellular  organisms,  as  we  shall  presently  see,  the  different  cells 
have  different  functions  to  perform,  and  the  cells  that  make 
up  the  individual  are  not  all  alike  as  they  are  in  the  forms 
already  described.  We  must  look  upon  the  Pandorina  and 
Ulothrix  as  intermediate  between  the  unicellular  and  the 
multicellular  forms.  In  this  way  they  illustrate  the  general 

*  Sometimes,  however,  they  do  grow  into  very  short  threads  which  are 
much  smaller  than  the  original. 


CELLULAR  STRUCTURE  OF  ORGANISMS  95 

biological  principle  that  sharp  lines  dividing  groups  can  hardly 
ever  be  drawn,  and  it  is  almost  always  possible  to  find  inter- 
mediate forms  connecting  widely  separate  types. 

True  Multicellular  Organisms. — Multi cellular  organisms  are 
always  made  up  of  more  than  one  cell;  but  the  fact  that  they 
consist  of  many  cells  is  not  enough  to  define  them  accurately. 
A  brief  account  of  the  manner  in  which  multicellular  organisms 
develop  will  explain  the  meaning  of  the  term.  In  all  cases 
they  begin  as  a  single  cell,  Which  may  be  either  an  egg  or  a 
spore.  This  cell  divides  into  two  parts,  these  into  four,  and 
so  on,  the  number  of  cells  increasing  indefinitely;  but  after 
dividing,  the  cells  remain  attached  instead  of  separating. 
After  a  while  some  of  the  cells  assume  a  variety  of  types,  i.e., 
they  become  differentiated  in  form  and  function,  and  play 
different  parts  in  the  life  of  the  organism.  Such  a  differentia- 
tion of  cells  occurs  in  all  true  multicellular  organisms.  Hence 
we  may  define  a  multicellular  organism  as  one  composed  of  many 
cells  which  show  a  differentiation  in  structure  and  function. 

With  this  differentiation  of  cells,  tissues  appear  for  the  first 
time.  Cells  with  similar  structure  and  function  are  commonly 
grouped  together,  to  form  a  tissue.  The  cells  with  special 
contractile  power,  for  example,  form  muscle  tissue;  cells  with 
power  to  secrete  bone  form  bony  tissue;  and  those  in  which 
conductility  and  irritability  are  particularly  developed  are 
grouped  together  to  form  nervous  tissue;  and  so  on.  Tissues  are, 
of  course,  impossible  among  unicellular  organisms,  but  univer- 
sal among  multicellular  organisms. 

With  the  multiplication  of  cells  and  their  differentiation, 
there  also  appears  the  formation  of  true  organs.  Among  the 
unicellular  animals  and  plants  there  may  be  certain  parts 
of  the  cell,  like  the  mouth  and  nucleus,  set  apart  for  certain 
functions,  and  these  are,  to  be  sure,  cell  organs.  But  they  are 
not  organs  in  the  sense  in  which  the  term  has  been  used  among 
the  multicellular  animals,  where  groups  of  cells,  usually  of 
various  kinds,  are  aggregated  to  form  distinct  parts  with 


96  BIOLOGY 

definite  functions,  so  that  an  organ  contains  several  tissues 
grouped  together  to  form  a  complex  structure. 

In  the  study  of  multicellular  organisms,  which  follows  in  the 
later  chapters,  it  will  be  seen  that  some  of  them  have  only  a 
few  simple  organs,  while  others  have  many  complex  organs. 
Those  which  are  of  simple  structure  and  have  few  organs  we 
call  low  organisms,  while  by  high  organisms  we  refer  to  those 
whose  structure  is  complex. 

PEN1CILLIUM,  A  SIMPLE  MULTICELLULAR  PLANT 

As  an  example  of  a  multicellular  plant  with  very  slight 
complexity,  we  will  study  one  of  the  common  molds,  which 
may  be  found  growing  upon  almost  any  moist  food  the  world 
over.  It  may  usually  be  obtained  in  abundance  by  placing 
a  bit  of  bread  or  a  slice  of  lemon  in  a  dish,  covering  it  so  that 
it  will  be  kept  from  drying,  and  allowing  it  to  remain  in  a 
warm  place  for  a  few  days.  The  object  will  soon  become 
covered  with  a  mold  (Penidllium)  which  after  a  day  or  two 
assumes  a  greenish-blue  color.  This  organism  is  somewhat 
difficult  to  study  under  the  microscope  because  it  is  so  massed 
together  that  special  methods  have  to  be  taken  for  preparing 
the  specimens.  The  best  method  is  to  plant  some  of  the  spores 
upon  a  little  jelly  which  has  been  hardened  on  a  glass  slide, 
and  then  study  the  spores  under  the  microscope  every  day 
and  notice  the  method  by  which  they  sprout  and  eventually 
form  the  complete  plant. 

Structure. — The  structure  of  Penidllium  may  best  be  under- 
stood by  studying  Figure  42.  It  is  made  up  of  a  mass  of  deli- 
cate, branching  threads,  extending  in  various  directions.  These 
threads  are  white  or  colorless  and  very  minute.  In  the  com- 
mon species  of  Penidllium  they  are  hardly  visible  to  the  naked 
eye,  although  in  some  species  of  molds  they  are  slightly  larger, 
and  in  others  they  are  large  enough  to  be  plainly  seen.  These 
threads,  which  are  known  as  the  mycelium  (Gr.  mykes  =  fun- 
gus), have  the  function  of  assimilation,  and  absorb  nourish- 


CELLULAR  STRUCTURE  OF  ORGANISMS 


97 


ment  from  the  substance  upon  which  the  molds  are  growing. 
Although  the  threads  are  very  delicate,  they  can  by  growth 
force  their  way  through  the  substance  upon  which  they  are 
feeding  until  they  penetrate  into  the  bread,  or  slice  of  lemon, 


FIG.  42. —  VARIOUS  MOLDS 

A,  a  colony  of  Penicillium,  showing  the  fruiting  spore-bearing  masses  arising  from  the 
mycelium;  B,  a  bit  of  the  colony  more  highly  magnified;  C,  one  of  the  fruiting  masses,  form- 
ing spores;  D,  a  colony  of  Mucor;  E,  the  sporangia  of  Mucor,  with  the  spores  emerging,  and 
showing  also  the  mycelium  below  not  divided  into  cells;  F,  a  bit  of  the  colony  of  Asper- 
gillus,  showing  a  third  method  of  formation  of  spores. 

or  decaying  apple,  for  some  distance,  and  the  material  thus 
becomes  permeated  with  the  mycelium.  Careful  study  of  the 
threads  of  this  mycelium  with  a  high  magnifying  power  shows 
that  they  are  made  up  of  many  cells.  Cross  partitions  divide 
the  threads  at  intervals  and  separate  the  consecutive  cells; 
Fig.  42  B.  The  contents  of  each  cell  include  protoplasm 
and  a  nucleus.  There  is  no  differentiation  of  the  cells,  all  in 


98  BIOLOGY 

the  mycelium  being  essentially  alike,  although  a  single  plant 
may  contain  hundreds  of  these  cells  in  its  growing,  branching 
mycelium. 

Reproduction. — The  only  noticeable  differentiation  of  cells 
that  is  seen  in  Penidllium  occurs  after  the  plant  has  grown 
for  a  few  days  and  is  ready  for  multiplication.  There  may  then 
be  seen  arising  from  the  mycelium  minute  branches  that  extend 
vertically  into  the  air  instead  of  growing  horizontally  over  the 
surface  of  the  object  upon  which  the  mold  is  nourishing  itself. 
These  rise  from  the  mycelium,  simply  as  branches,  and  are 
known  as  aerial  hyphae  (Gr.  hyphe  =  web) ;  Fig.  42  B  and  C. 
The  ends  of  these  hyphae  branch  into  a  number  of  finger-like 
processes,  which  extend  vertically,  parallel  with  each  other,  as 
shown  at  C;  after  a  time  these  branches  divide  by  constriction 
into  rows  of  minute  balls.  These  little  spheres  eventually  break 
off  from  the  plant  and  then,  blown  by  the  wind,  are  scattered 
far  and  wide.  Each  of  them  is  capable,  under  proper  conditions 
of  jnoisture  and  temperature,  of  developing  into  a  new  plant. 
They  are  evidently  spores,  this  particular  kind  of  a  spore 
being  named  conidia  (Gr.  konis  =  dust).  The  conidia  are 
bluish  in  color  and  they  cause  the  mold,  which  is  at  first  white, 
to  assume  a  distinct  blue  tinge,  giving  to  this  plant  its  common 
name  of  blue  mold.  They  are  extremely  light  and  may  be 
blown  for  a  long  distance  before  settling  to  the  ground.  When- 
ever they  do  settle  upon  any  moist  place  they  germinate; 
each  spore  produces  a  new  thread  which  in  the  course  of  a 
few  days  becomes  a  new,  branching  mycelium  and  thus  forms 
a  new  mold.  The  conidia  produced  by  a  single  plant  are  very 
numerous  and  so  light  that  they  may  be  carried  for  a  long  time 
in  the  air.  Indeed,  the  air  is  at  all  times  more  or  less  filled 
with  them,  in  summer  and  winter  alike;  and  it  follows  that 
any  moist  material  which  will  furnish  them  with  food,  like 
bread,  or  pieces  of  lemon,  or  the  surface  of  any  fruit,  if  exposed 
to  the  air  for  a  short  time,  will  be  sown  with  these  little  spores, 
and  in  a  few  days  will  begin  to  show  signs  of  molding.  So 


CELLULAR  STRUCTURE  OF  ORGANISMS  99 

widely  scattered  are  these  floating  mold  spores  that  it  is  hardly 
possible  to  expose  any  moist  organic  substance  even,  for  a 
few  minutes,  without  its  becoming  inoculated  with  some  of 
them  and  showing,  a  few  days  afterwards,  the  growth  of  molds 
upon  its  surface. 

Penidllium  has  a  second  method  of  multiplication  which  is 
rarely  seen.  It  occurs  only  under  special  conditions  which  are 
not  understood,  and  it  has  not  been  observed  by  many  bota- 
nists. It  consists  in  the  formation  of  minute  sacs,  within  which 
spores  are  formed,  usually  four  or  eight  in  number.  These 
sacs  are  known  as  asci  and  the  spores  are  ascospores.  Even- 
tually the  sacs  burst,  the  spores  come  out  and  are  then  capable 
of  developing  into  new  plants.  This  method  of  forming  spores 
is  evidently  similar  to  that  already  described  in  Yeast  (see 
Fig.  32  s),  and  shows  that  yeast  is  closely  related  to  the 
molds.  The  same  method  of  spore  formation  is  found  in  a 
large  number  of  other  plants  (lichens,  cup  fungi,  etc.)  and  is 
used  as  a  basis  of  classification  for  a  class  of  Fungi  called  As- 
comycetes  (Gr.  ascus  =  sac  +  mykes  =  fungus).  It  must  be 
no^ed,  however,  that  not  all  of  the  molds  form  spores  in  this 
way.  The  one  shown  in  Figure  42  D  has  a  method  of  repro- 
duction by  conjugation. 

OTHER  SPECIES  OF  MOLDS 

Molds  are  very  abundant  in  all  parts  of  the  earth  wherever 
there  is  much  moisture,  and  any  bit  of  organic  material  left 
to  itself  will  be  sure  to  show  signs  of  their  growth  in  course 
of  time.  Many  species  of  molds,  which  to  the  naked  eye 
closely  resemble  each  other,  may  be  distinguished  by  careful 
microscopic  study.  In  all  cases  the  plant  is  a  branching, 
colorless  mycelium,  similar  to  that  described  in  Penidllium. 
In  a  few  species,  however,  the  mycelium  is  not  divided  into 
cells  by  partitions,  as  in  Penicillium,  but  the  whole  thread 
forms  one  continuous  mass  called  a  syncytium;  Fig.  42  E. 
The  chief  method  by  which  the  molds  are  distinguished  from 


100  BIOLOGY 

each  other  is  not  by  the  structure  and  shape  of  the  mycelium, 
but  rather  by  their  method  of  producing  spores.  Penidllium 
is  one  of  the  more  common,  but  there  are  many  other  species 
in  which  the  spores  are  produced  by  different  methods.  Three 
of  these  methods  of  spore  formation  are  shown  in  Figure  42 
C,  E,  and  F.  In  some  cases  the  spores  are  formed  in  a  sac 
called  a  sporangium,  as  at  E.  In  others  they  are  borne  upon 
a  globular  head,  not  inclosed  in  a  sac;  see  F.  Other  species 
show  various  methods;  but  in  all  cases  the  method  of  spore 
formation  is  quite  distinctive,  and  a  careful  microscopic  study 
of  the  different  forms  makes  it  possible  to  separate  them  into 
species  according  to  their  methods  of  producing  spores. 

Molds  play  a  very  important  part  in  the  life  processes  in 
nature.  The  term  mold  is  not  a  proper  scientific  designation 
for  these  plants,  but  a  popular  name,  covering  a  variety  of 
plants  of  similar  form  and  structure,  but  with  many  different 
botanical  relations.  That  they  belong  to  different  groups  is 
proved  by  the  fact  already  mentioned  that  they  have  different 
methods  of  reproduction,  some  of  them  forming  ascospores, 
while  others  form  spores  by  a  process  of  conjugation,  which, 
as  we  shall  learn  later,  is  a  type  of  sexual  reproduction. 


LABORATORY  WORK 

The  laboratory  work  that  can  be  done  by  an  elementary  class  upon 
karyokinesis  is  very  limited.  Mounted  preparations  should  be  furnished 
by  the  instructor.  For  this  purpose  the  young  growing  root  tips  of  Podo- 
phyllum  are  excellent.  If  these  are  collected  in  the  spring  and  carefully 
preserved,  sectioned,  and  stained  in  iron  haematoxylin,  they  will  show  all 
stages  of  cell  division.  Longitudinal  sections  are.  best,  and  they  should  be 
studied  with  a  1/12  immersion  objective  to  make  out  the  details.  By 
patient  study  of  a  few  sections  thus  prepared  the  various  steps  in  karyoki- 
nesis may  be  made  out. 

If  the  instructor  can  furnish  other  examples  of  dividing  cells  the  student 
should  make  comparisons.  Many  tissues  of  animals  and  plants  may  be 
utilized. 


CELLULAR  STRUCTURE  OF  ORGANISMS  101 


Carchesium. — Colonial  forms  of  Vorticella-\ik.e  organisms,  either  Car- 
chesium  or  Zoothamnium,  may  usually  be  found  in  aquaria  in  which  various 
fresh- water  plants  are  kept.  The  dishes  which  have  been  prepared  for  the 
culture  of  Amceba  and  Paramecium  will  frequently  show  them.  If  they  are 
obtainable  they  should  be  studied.  No  special  methods  are  necessary,  the 
colonies  being  small  enough  to  be  placed  under  a  cover  glass  and  studied 
alive.  Staining  with  methylene  green  is  useful  to  bring  out  the  nuclei. 

Ulothrix  or  Spirogyra. — One  of  these  forms  should  be  studied  as  an 
example  of  filamentous  plants.  Either  of  them  may  be  found  in 
ponds  or  ditches  by  the  roadside.  They  are  to  be  studied  without  any 
special  preparation,  the  fresh  form  showing  most  points  perfectly  well. 
The  shape  of  the  cells  and  of  the  chlorophyll  bodies  should  be  noticed. 
The  nucleus  may  usually,  though  not  always,  be  seen  without  any  treat- 
ment. A  little  glycerine  added  underneath  the  cover  glass  will  cause  the 
protoplasm  to  contract  from  the  cell  walls.  Staining  with  methylene  green 
will  show  the  nucleus  if  it  has  not  been  seen  without  this.  If  material  is 
at  hand  to  show  the  conjugation,  it  is  desirable  to  have  the  student  study 
threads  of  conjugating  Spirogyra  and  compare  with  the  conjugation  of 
Paramecium  described  in  the  text.  The  reproduction  of  Ulothrix  by  for- 
mation of  spores  is  so  difficult  to  obtain  that  it  is  impractical  to  furnish 
material  to  a  class  for  study. 

Penicillium  and  Other  Molds. — Molds  may  be  easily  obtained  by  allowing 
bits  of  lemon,  banana,  bread,  etc.,  to  remain  for  a  few  days  in  a  closed  jar 
in  a  warm  place.  The  general  appearance  of  the  molds  can  be  studied  on 
the  surface  of  these  articles.  For  a  more  careful  study  it  is  necessary  to 
study  the  colonies  growing  from  spores.  A  simple  method  is  as  follows: 
Prepare  a  culture  medium  from  dried  beans  by  placing  a  pint  in  about  twice 
as  much  water  as  is  necessary  to  cover  them.  Allow  to  stand  12  hours  and 
add  enough  water  just  to  cover  the  beans.  Then  strain  off  the  liquid  from 
the  beans  and  filter.  To  the  filtrate  add  1%  of  agar  and  boil  so  as  to 
completely  dissolve  the  agar.  Place  the  material  in  test  tubes,  about  10 
c.  c.  in  each,  and  plug  the  mouths  of  the  tubes  with  cotton.  Place  in  a  wire 
basket  and  sterilize  by  steaming  for  three-quarters  of  an  hour  on  three 
successive  days.  To  use  this  culture  medium,  melt  several  of  the  tubes 
of  agar  and  pour  each  into  a  petri  dish,  allowing  the  agar  to  harden.  When 
thoroughly  hard,  remove  with  a  platinum  needle  a  minute  quantity  of 
the  spores,  which  appear  on  the  mold  on  the  lemon  or  bread,  and  just  touch 
the  surface  of  the  agar  with  the  spore-laden  needle  tip  in  several  places. 
This  will  sow  the  spores.  Place  the  petri  dish  (covered  to  prevent  drying) 
in  a  warm  place.  This  dish  may  then  be  studied  from  day  to  day  by 
putting  it  under  a  microscope,  and  the  sprouting  of  the  spores,  the 


102  BIOLOGY 

' 

growth  of  mold  colonies,  and  their  production  of  spores  can  be  followed 
in  detail.  Several  kinds  of  mold  will  usually  start  to  growing  on  the  lemon 
etc.,  and  may  be  distinguished  by  their  color.  The  different  species 
will  show  differences  in  spore  formation.  Sketches  of  the  colonies  and  their 
method  of  spore  formation  should  be  made.  The  type  which  will  be  most 
commonly  found  are  Penicillium,  Aspergillus,  and  Mucor;  Fig.  42. 


CHAPTER   V 


THE    CASTOR   BEAN,  A    COMPLEX 

PLANT 

THE  plants  hitherto  mentioned  do  not 
possess  flowers  and  belong  to  what  are 
called  the  flowerless  plants  or  Cryptogams 
(Gr.  cryptos  =  concealed  +  gamos  =  mar- 
riage). As  an  example  of  the  higher 
multicellular  plants  we  will  describe  one 
of  those  producing  true  flowers,  i.  e.,  one 
of  the  flowering  plants  or  Phanerogams 
(Gr.  phaneros  =  open  +  gamos) .  For  this 
purpose  we  will  study  the  castor  bean. 

THE  CASTOR  BEAN  (RIC1NUS  COMMUN1S) 

The  castor  bean  (Ritinus  communis) 
is  the  plant  from  which  castor  oil  is  ob- 
tained; it  is  also  used  as  an  ornamental 
foliage  plant  on  account  of  its  large, 
beautiful  leaves.  Other  plants  may  serve 
for  this  study,  but  this  one  illustrates 
especially  well  the  structure  of  the  higher 
plants.  The  seeds  may  be  obtained  at 
seed  stores  and  will  readily  sprout  in 
moist  sawdust. 

GROSS  STRUCTURE 

Figure  43,  which  represents  a  young 
seedling  of  the  castor  bean  about  two 
weeks  old,  illustrates  the  general  struc- 
ture of  other  multicellular  plants,  since 
the  higher  plants  are  essentially  alike  in 

103 


MULTICELLULAR 
P-. 


FIG.    43.  — A    YOUNG 

SEEDLING  OF  THE 
CASTOR  BEAN,  THREE 
WEEKS  OLD 

s,  the  stem;  r,  the  roots; 
f,  expanded  seed  leaves; 
p,  permanent  leaves. 


104 


BIOLOGY 


this  respect.  It  consists  of  a  stem  connecting  two  expanded 
surfaces,  the  one  ending  in  the  leaves,  and  the  other  dividing 
under  the  soil  into  fine  rootlets  which  bear  root  hairs.  Plants 
obtain  their  food  partly  from  the  air  and  partly  from  the 
soil,  and  this  explains  why  they  expand  their  branches  into 
leaves  in  the  air,  and  their  roots  into  root  hairs  in  the  soil.  The 
stem  of  the  plant  serves  chiefly  as  a  connection  between  the 
leaf  and  the  root  and  as  a  support  for  the  branches  and  leaves. 

STRUCTURE  OF  THE  STEM 


The  structure  of  the  stem  may  best  be  understood  by  begin- 

cross   section   of   a   young 
stem,  shortly  after  it  has 


ning   with  the  examination   of 
ep 


emerged    from   the    seed; 
see  Fig.  44.     * 

Fundamental  Cells.  - 
The  bulk  of  the  stem 
consists  of  a  mass  of  ap- 
proximately round  cells, 
which  are  called  funda- 
mental cells,  p.  These 
cells  are  largest  toward 
the  center  of  the  stem 
and  grow  smaller  toward 
the  outer  edge.  The  large 
cells  in  the  center  form  the  pith.  On  the  outer  edge  of  the 
stem  is  a  single  layer  of  small  rounded  cells  forming  the  epi- 
dermis (Gr.  epi  =  upon  +  derma  =  skin),  ep.  Just  beneath 
the  epidermis  are  several  irregular  rows  of  cells,  larger  than 
the  epidermal  cells,  known  as  the  cortex  (Lat.  cortex  =  bark), 
co.  At  this  stage  the  cortex  on  its  inner  edge  is  not  very  sharply 
marked  off  from  the  cells  which  fill  the  center  of  the  stem, 
and  form  the  pith. 

Fibrovascular  Bundles. — A  short  distance  within  the  cortex 
will  be  found  several  groups  of  especially  marked  cells,  }b, 


FIG.  44. —  A  SECTION  ACROSS  THE  STEM 

OF  THE    SEEDLING 

fb,  the  fibrovascular  bundle;  co,  the  cortex; 
ep,  the  epidermis;  p,  the  general  fundamental 
cells. 


THE  CASTOR  BEAN 


105 


known  as  fibrovascular  bundles  (Lat.  fibra  —  fiber  +  vas  = 
vessel).  In  the  young  stem  there  is  a  row  of  eight  to  ten  of 
these  groups,  arranged  to  form  a  ring  a  short  distance  beneath 
the  epidermis.  The  bundles  do  not  actually  touch  each  other, 
but  the  cells  of  the  pith  and 
the  cortex  are  connected.  v;  i-.i  <  \9° 

Structure  of  a  Fibrovas- 
cular Bundle.' —  Figure  45 
shows  a  highly  magnified 
view  of  a  cross  section  of 
one  of  these  fibrovascular 
bundles.  It  consists  of  three 
parts :  — 

1.  Running    across    the 
middle  are  several  rows  of 
small    thin-walled    cells 
known    as     the    cambium 
layer,  c  (Lat.  cambire=  to 
exchange).     These  cells  are 
full    of    active   protoplasm 
and  are  the   chief  growing 
cells  of  the  stem. 

2.  On  the  inside  of  this 
layer,  and  therefore  toward 
the  pith,  is  the  xylem,  x  (Gr. 
xylon  =  timber),  a  somewhat 


FlG.    45. A    HIGHLY    MAGNIFIED    SEC- 
TION  OF  A   FIBROVASCULAR   BUNDLE 


s,  sieve  cells; 
t,  tracheids; 
x,  is  the  xylem; 
ph,  the  phloem  part  of 
the  bundle; 


a,  accompanying  cells; 

c,  cambium  layer; 
co,  cortex; 

d,  ducts; 

pa,  parenchyma; 

st,  stereome  cells. 

triangular  mass  of  cells,  the  walls  of  which  are  thicker  than 
those  of  the  cambium.  Among  them  may  be  seen  at  least  two 
kinds  of  cells;  one  of  small  size  but  with  very  thick  walls 
forming  the  tracheids  (Gr.  trachea  =  windpipe)  or  wood  cells, 
t,  and  the  other  of  larger  size  with  relatively  thin  walls, 
forming  the  ducts  or  vessels,  d. 

3.  On  the  outside  of  the  cambium,  and  therefore  toward 
the  epidermis,  is  a  somewhat  irregular  mass  of  cells  called  the 
phloem  (Gr.  phloios  =  inner  bark),  ph,  within  which  may  be 


106 


BIOLOGY 


cost 

\ 


seen  four  kinds  of  cells.  There  are  a  few  large  cells  called  sieve 
cells,  Sj  and  near  them  some  small  cells  called  the  accompany- 
ing cells,  a.  Other  cells  still  smaller  and  with  thin  walls  form 
the  parenchyma  (Gr.  para  =  beside  +  en  =  in  +  chein  =  to 
pour),  pa,  and  a  few  cells,  with  very  thick  walls,  are  called 
the  stereome  cells  (Gr.  stereos  =  solid),  st.  The  cells  of  the 
cambium  do  most  of  the  growing;  as  they  multiply  they  pro- 
duce new  cells  both  on  their  inner  and  their  outer  edge,  causing 
the  bundles  to  increase  in  thickness  by  additions  between 
the  xylem  and  the  phloem. 

Figure  46,  a  longitudinal  section  through  a  bundle,   shows 
the  real  shape  of  the  cells.     The  cambium  layer  is  composed 

of     slightly    elongated 

?  '   f  /?      &•        SP  f  P        ce^s  w^h  scluare  ends. 

Each  of  these  cells  con- 
tains protoplasm  and  a 
prominent  nucleus,  dif- 
fering in  this  respect 
from  the  majority  of 
the  cells  of  the  bundle, 
which  are  empty  arid 
represent  only  the  cell 
walls  from  which  the 
protoplasm  has  been 
removed.  The  xylem 
cells  of  the  bundle, 
forming  the  wood 
proper,  show  several 
types.  The  large  ducts 
have  peculiarly  marked  cell  walls.  Some  of  them  show  rings 
forming  thickenings  on  the  inside  of  the  cell  wall,  or  the 
thickenings  may  take  the  form  of  a  spiral,  sp.  Other  ducts 
show  dots  or  pits  and  various  peculiar  markings,  d.  The 
smaller  cells,  the  tracheids,  t,  are  much  narrower  than  the 
ducts,  but  have  relatively  thicker  walls.  Some  have  square 


Phloem  Xylem 

FIG.  46. —  LONGITUDINAL  SECTION  OF  A 
FIBRQVASCULAR  BUNDLE 


a,  accompanying  cells; 

c,  cambium  cells; 
co,  cortex; 

d,  ducts; 
p,  pith; 


s,  sieve  cells; 

sp,  spiral  ducts; 

st,  sterome  cells; 

t,  tracheids  or  wood  cells. 


THE  CASTOR  BEAN 


107 


ends  and  others  have  ends  tapering  to  a  point,  the  cells  dove- 
tailing to  form  the  hard,  resisting  part  of  the  stem;  the  phloem 
outside  the  cambium  layer  also  contains  several  kinds  of  cells. 
Some  of  them  are  large  and  have  oblique  ends  which  are  per- 
forated by  apertures  that  place  one  cell  in  communication 
with  the  next  above  and  below.  Because  of  these  openings, 
these  cells  are  called  sieve  cells.  It  is  through  these  cells  that 
the  food  supply  is  transported  through  the  plant  from  the 
leaves.  Close  to  the  sieve  cells  are  smaller  cells,  the  accom- 
panying cells,  a,  which  are  long  and  slender.  The  phloem 
also  contains  many  rather  narrow  cells  with  square  ends  called 
parenchyma  cells,  and  a  few  small,  short  cells  with  very  thick 
walls  known  as  stereome  cells. 

The  same  longitudinal  sec- 
tion shows  that  the  pith,  p, 
is  made  of  short,  square  cells 
with  very  thin  walls.  Evi- 
dently the  pith  is  a  soft  tis- 
sue and  the  strength  of  the 
stem  is  due  to  the  hard  and 
resisting  fibrous  cells  in  the 
bundles.  Outside  of  the 
bundles,  directly  beneath  the 
epidermis,  it  will  be  seen  that 
the  cells  of  the  cortex,  co,  are 
much  like  those  of  the  pith, 
hardly  longer  than  they  are 
broad,  with  thin  walls  and 
square  ends. 

The  relation  of  the  fun- 
damental cells  to  the  fibro- 
vascular  bundles  is  better 
shown  in  Figure  47,  which  shows  how  the  bundles  extend 
through  the  stem  and  strengthen  it.  The  bundles  evidently 
consist  of  very  different  material  from  that  found  in  the  pith 


FIG.  47. —  PERSPECTIVE  VIEW  OF  A 
PIECE  OF  A  YOUNG  STEM,  SHOW- 
ING THE  FIBROVASCULAR  BUNDLE 
EXTENDING  LENGTHWISE  IN  THE 
STEM  FOR  SUPPORT 

fb,  fibro vascular  bundle; 
P,  pith. 


co,  cortex; 
ep,  epidermis; 


108 


BIOLOGY 


and  cortex.  They  are  mostly  long,  narrow  cells  with  com- 
paratively thick  walls,  which  are  hardened  by  the  deposition 
of  woody  substance.  The  name  fibrovascular  is  appropriately 
applied,  since  they  are  principally  made  up  of  fibers  mixed 
with  vessels.  The  strength  of  a  stem  depends  upon  the  density 
of  these  bundles,  and  the  thickness  of  the  walls  of  the  tracheids. 
Of  all  this  mass  of  cells  only  a  few  are  filled  with  living 
protoplasm.  The  cambium  cells  are  always  alive  and  the  sieve 
cells  may  contain  protoplasm.  The  other  cells  contain  proto- 
plasm when  they  first  form,  but  when  they  are  fully  grown 
most  of  them  are  only  the  empty  cell  walls.  This  is  particularly 
true  of  the  wood  cells  of  the  xylem.  Protoplasm  is  more  usually 
found  in  the  phloem  and  the  cortex  than  in  the  true  wood. 

Arrangement  of  Bundles  in  an  Older  Stem. — An  examination 
of  a  slightly  older  stem  shows  that  the  bundles  increase  in 

width  and  finally  fuse. 
In  Figure  48  it  will  be 
particularly  noticed 
that  the  cambium  layer 
of  one  bundle  has  grown 
until  it  comes  in  con- 
tact with  the  cambium 
layer  of  the  next,  and 
thus  forms  a  cambium 
ring  extending  around 
the  stem  a  short  dis- 
tance within  the  cortex 
separating  the  outer 
portion  of  the  stem, 

which  is  now  called  the  phloem  or  bark,  from  the  inner  part, 
the  xylem,  or  wood  proper.  Later  the  other  parts  of  the 
bundles  fuse,  forming  a  complete  ring  of  woody  tissue  and 
a  complete  ring  of  bark  separated  by  the  cambium. 

Remembering  that  this  cambium  layer  is  made  up  of  actively 
growing  cells,  it  is  easy  to  see  how  a  stem  of  this  kind  may 


fo  c    co 

FIG.  48. —  CROSS  SECTION  OF  AN  OLDER 
STEM,  SHOWING  CAMBIUM  FUSED  TO  FORM 
A  COMPLETE  RING,  C 

(In  other  respects  as  in  Fig.  43.) 


THE  CASTOR  BEAN 


109 


FIG.  49. —  DIAGRAM  SHOWING  THE  METHOD 
BY  WHICH  THE  CAMBIUM  LAYER  PRODUCES 
WOOD  CELLS  ON  ITS  INSIDE  AND  BARK 
CELLS  ON  THE  OUTSIDE 

be,  the  cells  of  the  bark; 
c,  cambium  cells; 
we,  the  wood  cells. 


increase  in  size.  As  the  cells  of  the  cambium  layer  divide, 
new  cells  are  formed  between  the  bark  and  the  wood  of  the  old 
bundles.  Some  of  these  new  cells  are  formed  inside  of  the 
cambium  layer,  and  outside  of  the  xylem,  as  shown  diagram  - 
matically  in  Figure  49. 
Other  cells  are  formed 
on  the  outside  of  the 
cambium  and  inside 

C 


of  the  old  phloem 
layer.  These  new 
cells  soon  assume  the 
form  of  new  wood 
cells,  new  tracheids  or 
ducts  on  the  inside; 
while  those  outside 
the  cambium  assume 

the  form  of  sieve  cells,  parenchyma,  etc.  It  thus  comes  about 
that  the  plant  is  producing  new  wood  cells  in  the  form  of  a 
layer  outside  the  old  wood  ring,  and  new  phloem  cells  in  a 
layer  inside  the  old  phloem  ring.  The  wood  grows  by  addi- 
tions upon  its  outer  surface  and  the  bark  by  additions  to  its 
inner  surface.  Since  the  cambium  forms  a  complete  ring, 
this  method  of  growth  evidently  will  produce  a  complete  ring 
of  wood  around  the  stem,  and  since  the  cambium  cells  con- 
tinue to  produce  new  cells  during  the  whole  of  their  active 
life,  they  will  continue  to  add  new  layers  of  wood  on  the  out- 
side of  the  old  wood.  The  wood  ring,  which  at  first  is  only 
a  thin  layer  just  inside  the  cambium,  becomes  thicker  and 
thicker  as  the  growth  continues.  As  it  becomes  thicker,  the 
stem,  of  course,  increases  in  diameter,  and,  since  the  cambium 
always  remains  on  the  outside  of  the  wood,  the  stem  may  keep 
increasing  in  size  as  long  as  the  cambium  cells  are  able  to  de- 
velop new  cells  to  be  deposited  as  wood  cells  on  the  outside 
of  the  old  wood.  In  the  same  way  the  cambium  deposits 
masses  of  cells  on  the  inner  side  of  the  phloem  of  the  bundles, 


110 


BIOLOGY 


and  the  bark  also  increases  in  thickness  by  growing  on  its 
inner  side.  This  growth  is,  however,  not  so  vigorous  as  is  that 
of  the  wood,  and  the  bark  does  not  increase  in  thickness  so 
much  as  does  the  stem.  Since  too  the  new  cells  of  the  bark 
are  deposited  on  the  inner  side,  the  older  parts  of  the  bark 
must  stretch  to  cover  the  increasing  diameter  of  the  growing 
stem.  When  a  stem  becomes  of  considerable  size  the  outer 
bark  will  be  found  to  be  rough  and  broken  by  the  expansion 
of  the  stem  which  it  covers. 

Some  plants,  which  have  but  one  year's  growth,  form  a 
single  ring  of  wood  as  described,  and  die  at  the  close  of  the 
season.  Other  plants,  like  large  trees,  do  not  die,  but  live 
year  after  year;  and  each  year  the  cambium  layer  adds  new 
masses  of  cells  outside  of  those  previously  existing.  In  plants 
that  live  in  regions  where  the  climate  changes  with  the  seasons, 

the  cells  formed  by  the 
cambium  layer  are  larger 
at  certain  seasons  of  the 
year  than  at  others.  In 
temperate  regions,  the 
wood  cells  formed  in  the 
spring  are  larger  and 
relatively  thinner  walled 
than  those  formed  later 
in  the  season.  During 
the  winter,  growth  ceases 
entirely;  but  as  soon  as 
spring  comes  again,  a 


FIG.  50. —  SECTION  ACROSS  AN  EXOGENOUS 

STEM   OF   FOUR    YEARS'    GROWTH,    SHOW- 
ING THE   FOUR  RINGS  OF  WOOD 


new  layer  of  large  cells 
will  be  deposited  on  the 
outside  of  the  last  ring 
that  was  deposited  in 
the  fall.  The  result  of 

this  is  a  series  of  rings  easily  recognized  when  a  cross  section 
of  a  stem  is  made;  Fig.  50.    Since  each  ring  indicates  ordi- 


fe,  bark; 

c,  cambium  layer; 

w,  wo<xl  ring. 


THE  CASTOR  BEAN 


111 


narily  a  year's  growth,  the  age  of  the  plant  may  be  determined 
by  counting  the  number  of  rings.  Such  rings  are  rarely  visible 
in  the  bark,  although  the  bark  also  increases  in  thickness  by 
layers  added  to  its  inner  side. 

From  this  description,  it  is  evident  that  the  growing  part  of 
the  stem  is  the  cambium  layer  and  that  the  stem  of  the  plant 
is  capable  of  continuing  its  life  only  as  long  as  this  cambium 
layer  is  intact.  What  is  known  as  girdling  a  tree  consists  in 
cutting  a  ring  through  the  bark  around  the  tree  in  such  a  way 
as  to  destroy  entirely  the  bark  and  the  cambium  layer;  this 
effectually  kills  the  tree  because  the  cambium  layer  is  destroyed, 
and  unless  there  is  a  connection  of  living  cambium  between 
the  roots  and  the  leaves,  the  life  of  the  plant  cannot  be  main- 
tained. It  is  also  evident  why  the  bark  may  be  stripped 
away  from  the  wood 
of  the  tree  so  readily. 
The  inner  edge  of  the 
bark  comes  next  to  the 
cambium;  the  cambium 
cells  are  thin- walled, 
full  of  soft  protoplasm 
and  easily  broken,  and 
hence  the  bark  is  easily 
separated  from  the  rest 
of  the  tree  at  this  point. 

Medullary    Rays.  - 
The  cells  in  the  vascular 
bundle  extend  up  and 
down  the  stem.    There 
are,    however,    other 
cells  that  run  horizon- 
tally,  extending     from 
the  center  to  the  outer 
edge.    These  form  what  are  called  medullary  rays  (Lat.  medulla 
=  marrow);  see  Fig.  51.    They  probably  serve  for  the  trans- 


$   ph 

FlG.  51. —  A  PARTLY  PERSPECTIVE  VIEW, 
SHOWING  THE  RELATION  OF  THE  PARTS 
IN  THE  STEM  OF  AN  OAK 


c,  the  cambium  layer; 
7»,  medullary  rays; 
ph,  phloem; 


s,  stereome  cells; 
x,  xylem. 


112 


BIOLOGY 


ference  of  the  material  from  the  outer  part  of  the  stem  toward 
the  center,  or  the  reverse.  This  type  of  stem  is  called  an 
exogenous  stem  (Gr.  exo  =  outside  +  genes  =  a  producing) ,  a 
name  given  to  it  from  the  fact  that  it  grows  by  the  addition 
of  new  layers  of  wood  upon  its  outer  side.  Such  a  stem  may 
increase  enormously  in  thickness;  some  trees  live  for  many 
hundreds  of  years  and  become  several  feet  in  thickness. 

There  is,  however,  another  type  of  stem  which  has  a  different 
arrangement  of  the  fibrovascular  bundle.  This  is  shown  in 

cross  section  in  Figure  52, 
which  represents  a  corn- 
stalk. In  this  section  there 
is  no  ring  of  wood,  the  fibro- 
vascular bundles  are  scat- 
tered irregularly  through 
the  stem,  and  there  is  no 
bark  or  true  pith.  More- 
over, closer  examination  of 
these  fibrovascular  bundles 
shows  that  they  do  not 
have  any  distinct  layer  of 
cambium  cells.  As  a  result, 

they  have  no  growing  layer  and  are  not  capable  of  increasing  in 
size.  Such  a  stem  is  known  as  an  endogenous  stem  (Gr.  endon 
=  within  +  genes),  and  belongs  to  a  type  of  plants,  like  the 
grasses  and  bamboos,  that  grow  tall  and  slender.  Their  stems 
are  only  a  little  larger  at  the  bottom  than  at  the  top  and  do  not 
materially  increase  in  diameter.  This  type  of  stem  forms  a  totally 
different  group  of  plants  from  the  first,  differing  in  many  respects 
in  their  leaves  and  flowers,  as  well  as  in  their  stem  structure. 

STRUCTURE  OF  THE  ROOT 

The  structure  of  the  root  of  the  castor  bean  resembles  that 
of  the  stem,  with  some  noticeable  differences.  A  cross  section 
shows  that  the  cortex  is  very  much  thicker  than  it  is  in  the 


FIG.  52. — CROSS  SECTION  OF 
ENDOGENOUS  STEM 

ep,  the  epidermis;  /,  the  fundamental  cells; 
fb,  the  fibrovascular  bundles  scattered  indefi- 
nitely through  the  stem. 


THE  CASTOR  BEAN 


113 


stem,  and  there  is  also  a  layer  of  cells  on  the  inner  side  of  the 
cortex  known  as  the  endodermis;  Fig.  53.  Within  this  are 
the  fibrovascular  cells  fused 
together  and  showing  little 
definition  into  cambium 
layer  or  fibrovascular  bun- 
dles. The  pith  is  reduced 
to  a  few  cells  in  the  center 
of  the  root.  The  tip  of  the 
root  is  always  small  and 
delicate,  yet  it  must  force 
its  way  through  the  hard 


co,  the  cortex; 
ep,  epidermis; 
en,  endodermis; 


fb,  fibrovascular  bundle; 
rh,  root  hairs. 


To  protect  them  the 


'en      ep 

FIG.  53. —  CROSS  SECTION  THROUGH 

A  SMALL  ROOT      . 
soil.      The  end  of  the  root 
contains  delicate,    thin- 
walled,  growing  cells,  which 
would  be  injured  in  pushing  their  way. 
tips  of   the  roots  are  covered  with  what  is  known  as  the 

root  cap;  Fig.  54.  This  is  a 
mass  of  rather  hard  corky 
cells  which  covers  the  deli- 
cate growing  cells  and  pro- 
tects them  from  injury  as  the 
root  pushes  its  way  through 
the  compact  soil. 

On  the  outside  of  the  root- 
lets, chiefly  near  their  ends,  are 
the  most  important  structures 
connected  with  the  root,  the 
root  hairs;  Figs.  55  and  56. 
They  are  very  delicate  threads 
which  grow  out  of  the  side 
of  the  root  and  radiate  from 
it  into  the  soil.  Figure  56 
shows  a  more  highly  magnified  view  of  some  of  these  hairs, 
showing  that  it  is  a  single  cell  arising  from  the  epidermis  of 


FIG.  54. — A  SEC- 
TION THROUGH 
THE  TIP  OF  A  ROOT 

Showing  the  root  cap,  c. 


Showing  the 
abundance  of 
root  hairs. 


114 


BIOLOGY 


FIG.  56. — CROSS  SECTION  OF 
A  MINUTE  ROOT 

Showing  the  relation  of  the  root  hairs  to 
the  cells  of  the  root. 


the  root.  The  root  hairs  are  present  in  immense  numbers 
on  the  fine,  delicate  growing  root  tips,  and  grow  in  all  direc- 
tions into  the  soil.  They  are 
thus  brought  into  close  contact 
with  particles  of  soil  and  serve 
the  plant  as  an  organ  for 
absorbing  water.  All  of  the 
nutrition  that  a  plant  derives 
from  the  soil  is  drawn  through 
these  root  hairs,  which  are 
closely  connected  with  the 
cells  on  the  interior  of  the 
root;  so  that  liquids  absorbed 
by  the  hairs  pass  readily  into 
the  substance  of  the  root 

itself.  From  here  they  pass  from  cell  to  cell,  and  eventually 
find  their  way  to  all  parts  of  the  plant.  The  root  hairs,  con- 
stituting the  absorbing  organ  of  the  plant,  are  of  great  func- 
tional value.  If  a  plant  is  forcibly  pulled  out  of  the  soil,  all  of 
the  root  hairs  are  torn  from  the  root  and  left  attached  to  the 
particles  of  the  earth.  If,  however,  the  whole  plant  is  removed 
from  the  ground  and  the  soil  is  carefully  washed  from  the  roots, 
the  root  hairs  may  be  found  still  attached  to  the  rootlets,  and 
may  show  grains  of  sand  attached  to  the  root  hairs. 

STRUCTURE  OF  THE  LEAF 

A  complete  leaf  consists  of  three  parts:  The  broadly  ex- 
panded blade;  the  contracted  stem  or  petiole;  and  two  little 
appendages  called  stipules  attached  to  the  base  of  the  petiole 
where  it  is  connected  with  the  stem.  The  stipules  are  not 
present  in  all  leaves  and  are  not  found  in  the  castor  bean. 
Running  from  the  top  of  the  petiole  out  into  the  blade  are  a 
series  of  fine  veins;  in  some  plants  they  run  in  a  parallel  direc- 
tion (parallebveined  leaves),  and  in  others  they  branch  profusely 
into  many  small  twigs  (netted-veined  leaves). 


THE  CASTOR  BEAN 


115 


Minute  Structure  of  the  Leaf. — A  section  across  the  petiole 
of  a  leaf  shows  a  structure  similar  to  that  found  in  the  stem 
of  a  plant,  except  that  there  is  no  regular  ring  of  fibrovascular 
bundles  and  no  cambium  layer.  In  this  petiole  may  be  seen 
several  fibrovascular  bundles  separated  from  each  other;  and 
if  these  are  traced  down  to  the  stem  from  which  the  petiole 
of  the  leaf  arises,  they  will  be  found  continuous  with  the  fibro- 
vascular bundles  of  the  stem.  Followed  into  the  blade  of  the 
leaf,  these  bundles  are  found  to  pass  out  into  it  and  form  the 
veins.  Thus  the  veins  of  the  leaf  are  simply  an  extension  of 
a  few  of  the  fibrovascular  bundles  that  come  from  the  stem. 
Being  hard  and  tough,  they  give  sufficient  rigidity  to  the  leaf 
to  support  the  softer  parts,  which  are  the  active  portions  of 
the  leaf  structure. 

Microscopic  Structure  of  the  Blade. — A  cross  section  through 
the  blade  of  the  leaf  is  most  instructive,  since  it  is  in  the  blade 


ep- 


st 


FIG.  57. —  CROSS  SECTION  OF  A  BIT  OF  THE  BLADE  OF  A  LEAF 


cl,  chlorophyll  bodies; 

ep,  epidermis; 

fb,  fibrovascular  bundles; 


m,  mesophyll  cells; 
p,  palisade  cells; 
st,  the  stomata. 


of  the  leaf  that  the  most  important  function  of  plant  life  is 
carried  on.  Upon  its  upper  and  under  surface  there  are  single 
layers  of  cells,  the  epidermis;  Fig.  57  ep.  These  are  made  of 
small,  irregular  cells,  closely  compacted  together  and  possessing 


116 


BIOLOGY 


a  hard  cell  wall  forming  a  layer  that  is  impervious  to  liquids 

and  even  to  gases.     They  form  a  covering  of  the  leaf  which 

prevents  the  entrance  of  water,  and 
protects  it  from  too  great  a  loss  of 
water  by  evaporation.  Through  the 
epidermis  are  numerous  openings 
known  as  stomata  (Gr.  stoma  = 
mouth),  st,  that  serve  as  breathing- 
pores.  If  a  bit  of  the  epidermis  is 
stripped  from  the  leaf,  it  will  ap- 
pear as  shown  in  Figure  58.  The 
cells  of  the  epidermis  are  irregular 
in  shape,  due  to  the  irregular  growth 
of  the  leaf,  and  among  them  are  nu- 
merous pores.  Each  pore  is  sur- 
rounded by  two  crescent-shaped  cells, 
guard  cells,  so  related  to  each  other 
that  the  pore  itself  lies  between  the 
two  crescent  cells.  The  guard  cells 
are  capable  of  expansion  and  con- 
traction under  different  conditions. 
As  they  expand,  they  straighten  out 

and  close  the  opening  of  the  stomata;  and  when  they  contract 

they  shorten  slightly  and  the  opening 

of   the  stomata  is  enlarged;  Fig.  59. 

In  this  way  they  can  change  the  size 

of  the  breathing  pores  of  the  plant 

and  thus  regulate  the  amount  of  air 

that  passes  in  and  out  of  the   leaf. 

These  stomata  occur  in  the  epidermis 

of  the  petiole  and  all  over  the  leaf,  less 

abundantly  on  the  upper  side  than  on 

the  under  side.    In  the  leaves  of  water        oc<  suard  cells- 

plants,  however,  the  stomata  are  chiefly  on  the  upper  side  of  the 

leaves,  where  they  are  in  contact  with  the  air  when  the  plant 


FIG.    58. —  THE   EPIDERMIS 
SHOWING  THE  STOMATA 

A,  from  the  leaf  blade;  B,  from 
the  petiole. 


FIG.  59. —  DIAGRAMMATIC 
CROSS  SECTION  OF  A 

STOMA 


THE  CASTOK  BEAN  117 

floats  on  the  surface  of  the  water.  The  shape  of  the  stomata 
and  guard  cells  varies  slightly  in  different  plants,  but  their 
structure  is  always  essentially  like  that  seen  in  the  Figures 
58  and  59. 

In  the  middle  of  the  leaf  may  be  seen  cross  sections  of  the 
veins,  which  are  typical  fibrovascular  bundles  (Fig.  57  /&), 
composed  of  essentially  the  same  kind  of  cells  that  we  have 
found  in  the  bundles  of  the  stem.  The  rest  of  the  substance 
of  the  leaf  is  filled  with  a  loose  mass  of  cells  which  are  the 
active  cells  of  the  plant.  Immediately  under  the  upper  epi- 
dermis is  a  layer  of  slightly  cylindrical  cells  forming  a  fairly 
definite  row.  These  are  called  the  palisade  cells;  Fig.  57  p. 
They  contain  minute  granules  (chloroplasts)  of  green  coloring 
matter  called  chlorophyll  (Gr.  chloros  =  green  -f  phyllon  = 
leaf),  cl,  and  each  contains  protoplasm  and  a  nucleus.  Below 
the  palisade  cells  are  other  cells  more  irregular  in  shape  and 
more  loosely  packed.  In  this  part  of  the  leaf  these  cells  are 
called  mesophyll cells  (Gr.  mesos  =  middle  +  phyllon  =  leaf),ra, 
and  their  shape  is  so  irregular  and  they  are  so  loosely  packed 
that  many  air  spaces  communicating  with  the  exterior  through 
the  stomata  are  left  between  them.  These  mesophyll  cells 
are  filled  with  active  protoplasm  and  crowded  with  chloro- 
plasts (Gr.  chloros  =  green  +  plastos  =  molded) .  The  intimate 
connection  which  these  chlorophyll-bearing  cells  have  with 
the  air  that  enters  through  the  stomata  is  evident  from  Figure 
57,  and  is  a  matter  of  extreme  significance,  since  these  cells 
extract  from  the  air  the  food  from  which  the  plant  manufac- 
tures starch,  the  first  step  in  the  production  of  food  for  all 
animals  and  plants;  see  page  129. 

The  epidermis  of  the  leaves  of  some  plants  has  various 
other  structures.  Not  infrequently  it  is  prolonged  into  hairs 
of  various  shapes  and  sizes;  sometimes  these  hairs  have  a 
little  poison  at  their  ends  and  then  they  constitute  nettle  hairs. 
The  general  function  of  the  hairs  is  to  protect  the  plant  from 
injury  by  small  insects  and  other  animals. 


118 


BIOLOGY 


REPRODUCTIVE  ORGANS 

The  organs  which  are  designed  for  reproduction  are  widely 
different  in  different  groups  of  plants.  Among  the  higher 
plants  this  function  is  carried  on  by  specially  modified  branches 
known  as  flowers.  Although  the  greatest  variety  is  shown 
among  the  flowers  of  different  plants,  when  compared  they 
are  readily  seen  to  have  the  same  general  structure.  The  fol- 
lowing description  is  not  that  of  the  flower  of  the  castor  bean, 
or  of  any  other  plant,  but  an  ideal  description  of  a  typical 
flower,  and  in  a  general  way  applies  to  the  flowers  of  all  the 
higher  groups  of  plants. 

General  Structure  of  a  Flower. — A  flower  is  always  borne 
at  the  end  of  a  stem;  even  although  it  appears  to  come  from 

the  side,  when  carefully  exam- 
ined it  is  found  to  be  really 
on  the  end  of  a  short,  unde- 
veloped stem  arising  from  the 
side  of  the  larger  one.  Indeed, 
a  flower  is  itself  a  short  stem 
bearing  usually  four  rows  of 
leaves;  Fig.  60.  The  stem  of 
the  flower  is  called  the  pedun- 
cle, p;  at  its  top  it  is  fre- 
quently slightly  enlarged  to 
bear  the  several  rows  of 
leaves,  this  enlargement  being 
known  as  the  receptacle,  r. 
The  flower  itself  is  composed 
of  four  rows  of  leaves  so 
closely  attached  to  each  other 
that  they  appear  to  arise  at  the  same  point  of  the  stem; 
careful  study,  however,  shows  that  in  all  complete  flowers 
the  four  different  kinds  of  leaves  are  produced  one  row  above 
the  other. 


FIG.  60. —  DIAGRAM  SHOWING  THE 
PARTS  OF  AN  IDEAL  FLOWER 

r,  receptacle; 


a,  the  anther; 
car,  carpels; 
p,  peduncle ; 
pi,  petal; 


r,  recepta 
s,  sepals; 
st,  stamens. 


THE  CASTOR  BEAN 


lid 


The  Calyx  composed  of  Sepals.— The  lower  row,  which  is 
on  the  outer  side  of  the  flower,  is  made  up  of  small  parts  which 
are  usually  green  and  leaf-like  in  appearance.  This  row  is 
known  as  the  calyx,  and  the  leaves  of  which  it  is  composed 
are  called  sepals. 

The  Corolla  composed  of  Petals.— Just  above  and  within 
the  calyx,  in  an  ordinary  flower,  is  a  second  row  of  leaves, 
usually  larger  than  the  calyx  and  of  some  brilliant  color.  This 
row  of  leaves  is  known  as  the  corolla  and  the  individual 
leaves  as  petals,  pi.  It  is  these  colored 
petals  that  give  the  flower  its  brilliancy, 
and  their  function  seems  to  be  to  attract 
the  insects,  that  are  useful  to  the  flower 
in  producing  cross  fertilization;  see  page 
267.  The  calyx  and  corolla  together  are 
sometimes  known  as  the  perianth  (Gr.  peri 
=  around  -f  anthos  =  flower).  In  some 
flowers  either  the  calyx  or  the  corolla  may 
be  lacking,  and  in  others  both  may  be  lack- 
ing. When  only  a  single  row  of  leaves  is 
found  in  the  perianth,  it  is  customary  to  call 
it  a  calyx,  irrespective  of  its  shape  and  color, 
and  such  plants  are  usually  spoken  of  as  apeta- 
lous  (Gr.  a  =  without  +  petalon  =  a  leaf). 

The  Stamens.  — Within  the  petals  is  a  FIG.  61.— THREE 
third  row  of  leaves,  the  stamens,  st,  which, 
however,  have  almost  wholly  lost  their  re- 
semblance to  leaves.  Each  of  these  consists 
of  a  delicate  stem,  called  the  filament  (Fig. 
61),  at  the  top  of  which  are  little  sacs,  usu-  of  splitting  open  to  dia, 
ally  two  in  number,  which  are  known  as  the 
anther,  a.  Within  these  sacs  are.  produced  large  numbers 
of  spores,  the  spores  in  this  case  being  called  pollen;  Fig.  62. 
The  stamens  are  usually  as  many  as  the  petals,  although  in 
some  flowers  there  are  two  or  three  times  as  many,  and  in 


STAMENS  WITH 
D  I  F  F  ERE  NT 
FORMS  OF  AN- 
THERS 

a,  showing  methods 
splitting  open  tc 
charge  the  pollen. 


120 


BIOLOGY 


FIG.    62.  —  DETAILS   OF    AN 

ANTHER   OF  A   FLOWER 

A,  section  across  the  anthers, 
showing  the  four  cavities  with  the 
pollen,  p,  enclosed;  B  and  C,  pollen 
grains;  n,  nucleus;  sp,  the  pollen  cell 
or  microspore. 


others  some  of  the  stamens  disap- 
pear. Some  flowers  are  entirely 
without  stamens  and  are  spoken 
of  as  imperfect  flowers. 

Carpels. — Within  the  stamens  is 
the  fourth  and  last  row  of  leaves. 
In  this  case  the  parts  have  lost  all 
resemblance  to  leaves  and  in  ordi- 
nary flowers  they  would  never  be 
thought  of  as  corresponding  to 
leaves,  unless  carefully  examined. 
The  parts  of  this  inner  row  are 
known  as  carpels  (Gr.  carpos  = 
fruit);  Fig.  60  car.  Each  carpel 
consists  of  three  portions,  a  lower, 
somewhat  expanded  portion  known 
as  the  ovary  (Fig.  63  ov),  and  above 
this  a  more  or  less  elongated,  slen- 
der part,  called  the  style,  s,  whose 
upper,  slightly  roughened  surface 
is  known  as  the  stigma,  st.  These 
three  parts  form  what  is  commonly 
called  the  pistil.  It  frequently 
happens  that  the  number  of  car- 
pels is  less  than  that  of  the  calyx, 
corolla,  or  stamens.  Moreover,  the 
carpels  are  often  so  fused  together 
that  it  is  impossible  to  count  dis- 
tinctly the  separate  carpels  of  which 
it  is  composed.  When  this  occurs, 
there  is  found  in  the  center  of  the 
flower  what  is  known  as  a  com- 
pound pistil,  i.  e.,  a  pistil  made  of 

A,  a  pistil  made   up  qi  a  smgie 

Several    Carpels    fused    together;     See       carpel;  B,  a  compound  pistil  made 

up  of  three  carpels;  s,  the  style;  st, 

Fig.    63  B.     But  it    is   usually      '     " 


A  B 

FIG.  63.— PISTILS 

A,  a  pistil  made   up  of  a  single 


the  atigma;  ov,  the  ovary. 


THE  CASTOR  BEAN 


121 


easy  to  perceive  this  condition  in  the  pistil  and  to  determine 
the  number  of  carpels  of  which  it  is  made.  The  pistil  shown 
at  Figure  63  B  is  evidently  made  up  of  three  carpels,  with 
fused  ovaries,  but  remaining  more  or  less  separated  from 
each  other  above.  In  some  cases  the  style  and  stigmas,  as 
well  as  the  ovaries,  are  fused  together,  and  it  is  more  diffi- 
cult to  determine  the  number;  but  even  in  these  cases  we 
can  easily  distinguish  in  a  compound  pistil  the  number  of  car- 
pels of  which  it  is  composed,  by  counting  the  number  of  rows 
of  seeds  in  the  ovary,  there  being  usually  one  row  of  seeds 
for  each  of  the  carpels  in  the  compound  ovary. 

In  some  flowers  the  carpels  are  entirely  absent,  and  such  a 
flower  is  called  an  imperfect  flower.  A  perfect  flower  is  a 
flower  that  has  both  sta- 
mens and  pistils,  and  such 
a  flower  is  capable  of  pro- 
ducing seeds.  An  imper- 
fect flower  is  one  in  which 
either  the  stamens  or  the 
carpels  are  lacking,  and 
such  flowers  are  not  alone 
capable  of  producing 
seeds. 

Within  the  ovary  are 
found  the  true  reproduc- 
tive bodies.  These  at  first 
appear  as  several  rounded 
masses  called  ovules  (Fig. 
64),  within  each  of  which 
is  a  single  minute  spore 
cell,  s,  corresponding  to 
"the  spores  which  form  the 
pollen.  This  spore  never 
leaves  the  ovule,  but  undergoes  a  series  of  changes  within 
the  ovary  which  result  in  the  production  in  each  ovule  of 


FlG.  64. —  A  LONGITUDINAL  SECTION  OF 
A  PISTIL  IN  DIFFERENT  STAGES  OF 
DEVELOPMENT 

A,  showing  the  immature  ovules  with  the  en- 
closed spore,  s;  B,  the  older  ovules,  containing  an 
egg,  e;  C,  the  ripened  ovary  with  the  seeds,  sd, 
each  containing  a  young  embryo  plant. 


122  BIOLOGY 

one  or  two  eggs,  e.     As   these   spores   produce  eggs,  which 
are  the  female  reproductive  bodies,  we  may  speak  of  them  as 
female  spores.    Older  botanists,  before  their  real 
— -*\£      nature  was  understood,  called  them  by  the  name 
of  embryo  sacs.      The  small  spores  (pollen)  pro- 
duced  in   the    anther,    on  the    other  hand,  are 
spoken  of   as   male   spores,   inasmuch   as   their 
function  in  reproduction  is  that  of  the  male.* 

Fertilization. — The  pollen  grains,  or  male  spores 
from  the  anther,  are  carried  by  some  means  to 
the  stigma  of  the  stamen.  They  are  sometimes 
carried  by  insects,  sometimes  by  wind,  or  by 
various  other  means.  The  stigma  on  the  top 
of  the  pistil  is  usually  rough  and  sticky,  and  the 
pollen  grains  readily  adhere  to  it.  In  this  posi- 
tion, the  pollen  grows  and  a  long  tube  arises 
from  each  pollen  grain  and  pushes  its  way  down 
through  the  style  and  within  the  ovary;  Fig. 
65  pt.  This  tube  is  the  pollen  tube.  In  the 
FIG.  65.  —  meantime  the  female  spore  in  the  ovary  has  pro- 
LONGITUDI-  duced  the  egg.  The  pollen  tube  is  attracted  to 

NAL  SECTION     .,  -,    ~        ,?       .,       ,.  •  -,i 

the  egg,  and  finally  its  tip  comes  in  contact  with 
CARPEL  it-  Inside  of  this  pollen  tube  is  found  one  or 
showing  the  more  special  cell  nuclei  which  are  carried  in  the 
?achedntop'the  tip  of  the  growing  tube  and  finally  pass  into  the 
SKIS  poT  egg,  fusing  with  it.  This  latter  process  is  called 
whichbehap's  fertilization. 

The       The  Seed. — After  the  egg,   which  is  a  single 
cell,  has  fused  with  the  contents  of  the  pollen 
tube,  it  divides,  and  in  a  few  days  produces  a 
little  multiceilular  plant.     This  plant,  while  still  in  the  ovary  of 
the  pistil,  develops  a  stem  and  one  or  more  leaves;  Fig.  64  sd. 

*The  pollen  because  of  the  small  size,  is  also  called  a  microspore,  and 
the  spore  in  the  ovary,  being  larger,  is  called  a  megaspore  or  macrospore. 
The  significance  of  this  we  shall  notice  in  a  later  chapter. 


THE  CASTOR  BEAN 


123 


FIG.  66. —  LONGITUDINAL  SECTIONS 

OF  THREE    SEEDS 

Showing  the  enclosed  young  plant  or  embryo,  e, 
and  food,  /.   In  the  middle  figure,  the  food  is  deposited 


After  a  few  days  it  stops  growing  and  becomes  surrounded 
by  a  hard  shell,  and  is  now  known  as  a  seed;  in  this  form, 
protected  by  its  shell,  it  may  remain  dormant  for  some 
time.  If  any  seed  is  carefully  examined  it  will  be  found  to 
contain  a  little  plant,  or 
seedling,  with  a  stem  and 
one  or  more  leaves;  Fig. 
66.  The  leaves  inside  of 
the  seed  are  known  as  cot- 
yledons, and  while  they 
are  true  leaves  they  are 
different  in  shape  and 
structure  from  the  leaves 
which  this  same  plant 
is  to  produce  later  when 
the  seed  has  germinated; 
see  Fig.  43. 

There   is    also    deposited    in  the  leaves  of  the  embryo;  in  the  two  other  figures 

the  food  is  around  the  embryo. 

in  the  seed,  either  around 

the  seedling  or  within  it,  a  quantity  of  food  upon  which  the 
young  plant  can  feed  during  the  first  few  days  of  its  life,  before 
it  can  feed  itself  from  the  soil. 

This  whole  process  of  fertilization,  growth  into  a  little  plant, 
and  the  development  of  the  shell  around  it  to  form  a  seed, 
occurs  within  the  pistil  of  the  flower.  The  flower  in  the  mean- 
time withers  and  the  ovary  increases  in  size  to  accommodate 
the  growing  seeds.  Eventually,  the  fruit  is  broken  open 
(dehiscence)  and  the  seeds  drop  out.  When  this  occurs  the 
duty  of  the  flower  is  over  and  all  its  parts  decay,  leaving  the 
plant  without  flowers  until  the  next  season.  From  this  de- 
scription, it  will  be  seen  that  there  are  in  the  flower  at  least 
four  different  kinds  of  reproductive  bodies:  the  male  spores, 
or  pollen;  the  female  spores,  or  embryo  sac;  the  eggs  which 
develop  from  the  female  spores  and  finally  grow  into  seeds; 
and  the  male  nuclei  inside  the  pollen  tube  which  fuse  with  the 


124  BIOLOGY 

egg.  The  relation  of  these  different  bodies  to  one  another  and 
to  the  general  process  of  reproduction  will  be  considered  in  a 
later  chapter. 

LABORATORY  WORK  ON  THE  CASTOR  BEAN 

Seeds  may  be  obtained  at  almost  any  seed  store.  For  the  study  of  the 
seeds  they  should  be  soaked  over  night  in  water,  which  will  soften  them  so 
that  the  outer  covering  may  be  removed  and  the  seed  readily  dissected. 

The  study  of  the  plant  structure  should  be  made  from  young  seedlings. 
Soak  the  beans  in  water  over  night  and  then  plant  them  in  a  box  containing 
moist  sawdust,  covering  the  box  with  a  piece  of  glass  to  prevent  evapora- 
tion. Place  the  box  in  a  warm  place  and  water  the  seeds  daily,  keeping 
the  sawdust  quite  moist.  The  seeds  will  sprout  quickly  and  at  varying 
periods  of  growth  plants  may  be  removed,  the  sawdust  washed  from  their 
roots,  and  the  plants  studied  as  a  whole. 

For  the  study  of  the  stem  both  cross  sections  and  longitudinal  sections 
should  be  made  with  a  sharp  razor,  the  piece  of  the  stem  to  be  sectioned 
being  held  between  two  bits  of  pith  which  are  hollowed  out  to  receive  them. 
These  sections  may  be  mounted  in  water  and  studied  directly,  without 
any  further  preparation.  Some  points  can  be  seen  more  satisfactorily  by 
the  use  of  various  stains.  It  is  best  to  begin  with  the  study  of  a  young 
seedling  about  two  inches  high,  and  to  follow  with  older  plants  which  will 
show  the  growth  of  the  fibrovascular  bundles  and  their  fusion  into  a  ring. 
All  of  the  points  mentioned  in  the  text  should  be  studied. 

The  study  of  the  root  is  made  in  the  same  way.  To  obtain  root  hairs, 
it  is  better  to  sprout  sunflower  seeds  by  placing  them,  after  soaking  in 
water,  between  two  layers  of  blotting  paper  in  a  covered  dish,  which  should 
be  kept  moist  and  warm.  After  two  or  three  days  the  rootlet  of  the  young 
seedling  will  show  a  mass  of  root  hairs.  They  should  be  examined  through 
a  lens  without  disturbing  the  seedling,  and  then  one  of  the  rootlets  should 
be  placed  in  a  watch  glass  in  water  and  examined  with  a  microscope. 

The  epidermis  of  the  leaf  may  be  studied  by  stripping  off  with  fine  forceps 
a  bit  of  the  epidermis  from  the  upper  or  under  side  of  a  leaf.  Any  plant 
will  serve  for  this,  and  it  is  well  to  examine  the  epidermis  of  several  different 
plants.  The  study  should  be  made  with  a  high  power.  The  internal 
structure  of  a  leaf  must  be  made  by  cross  sections.  These  are  very  diffi- 
cult to  make,  and  prepared,  stained  sections  should  be  furnished  by  the 
instructor. 

The  stems  of  other  plants  showing  annual  rings  of  growth  should  alsc 
be  studied  in  both  cross  and  longitudinal  sections.  Twigs  of  the  pine, 


THE  CASTOR  BEAN  125 

apple,  or  oak  which  show  about  three  years'  growth  are  satisfactory.  The 
wood  is  hard  to  cut  and  is  apt  to  injure  the  razor.  It  may  be  softened  by 
soaking  the  stem  in  a  mixture  of  equal  parts  of  alcohol  and  glycerine.  The 
stems  should  remain  in  this  mixture  for  several  days  at  least,  and  may  be 
left  in  it  for  months  without  injury,  and  be  ready  for  section  at  any  time. 
For  the  study  of  a  flower  any  simple  wild-flower  may  be  used  to  show  the 
general  relations  of  the  reproductive  organs.  A  common  Trillium  is  an 
excellent  example.  The  grosser  anatomy  of  the  flower  should  be  studied; 
sections  should  be  made  through  the  ovary  both  of  a  young  flower  and,  if 
possible,  of  the  fruit  after  the  flowering  is  completed,  in  order  to  show  the 
chambers  of  the  ovary  and  the  seeds  with  their  attachments.  The  pollen 
should  be  examined  with  a  microscope. 

BOOKS  OF  REFERENCE 

ANDREWS,  Practical  Course  in  Botany,  American  Book  Company, 
New  York. 

ATKINSON,  College  Botany,  Henry  Holt  &  Co.,  New  York. 

BERGEN  and  CALDWELL,  Practical  Botany,  Ginn  &  Co.,  Boston. 

CALDWELL,  Plant  Morphology,  Henry  Holt  &  Co.,  New  York. 

COULTER,  BARNES,  and  COWLES,  Text-book  of  Botany,  American  Book 
Company,  New  York. 

CURTIS,  Development  and  Nature  of  Plants,  Henry  Holt  &  Co.,  New  York. 

DUGGAR,  Plant  Physiology,  The  Macmillan  Co.,  New  York. 

GANONG,  Plant  Physiology,    Henry  Holt  &  Co.,  New  York. 

MACDOUGALL,  Plant  Physiology,  Henry  Holt  &  Co.,  New  York. 

STEVENS,  Anatomy  of  Plants,  P.  Blakiston's  Son  &  Co.,  Philadelphia 

STRASBURGER,  NOLL,  SCHENCK,  and  KARSTEN,  Text-book  of  Botanyr 
The  Macmillan  Co.,  New  York. 


CHAPTER  VI 
THE  PHYSIOLOGY  OF  A  TYPICAL  PLANT 

IN  order  to  carry  on  its  life  a  plant  must  have  an  income  of 
matter  and  energy.  The  problem  of  energy  will  be  reserved  for 
a  later  chapter:  only  a  consideration  of  the  relation  of  plants 
to  their  food  and  its  utilization  will  be  given  here. 

Plant  Foods. — The  income  of  an  ordinary  green  plant  is  de- 
rived partly  from  the  air  and  partly  from  the  soil.  It  consists 
of:- 

1.  Carbon  dioxid  (CO2),  absorbed  from  the  air  by  the  leaves. 

2.  Water  (H20),  absorbed  from  the  soil  by  the  root  hairs. 

3.  Nitrates  or  other  nitrogen  salts,  absorbed  from  the  soil  by 

the  root  hairs. 

4.  Phosphates,    potash  salts,    and    other  minerals,   in   small 

amounts,  absorbed  from  the  soil  by  the  root  hairs. 

The  carbon  dioxid  and  water  are  absorbed  by  the  plant  in 
enormous  quantities  and  constitute  by  far  the  largest  proportion 
of  their  foods;  the  soil  minerals,  although  absolutely  necessary, 
are  needed  only  in  small  quantities.  Roughly  speaking,  the 
amount  of  material  absorbed  from  the  soil  is  represented  by  the 
ashes  that  are  left  after  a  plant  is  burned.  All  of  the  minerals 
are  dissolved  by  the  waters  in  the  soil  and  absorbed  in  this  form 
by  the  root  hairs. 

Ascent  of  Sap. — Since  the  foods  are  obtained  through  organs 
situated  at  the  opposite  ends  of  the  plant,  in  order  that  they 
may  be  utilized  they  must  be  brought  together,  and  since  it  is 
in  the  leaves  that  they  are  utilized,  the  water,  containing  the 
dissolved  minerals  absorbed  by  the  roots,  must  be  carried  up  the 
stem  to  the  leaves.  This  ascent  of  sap  is  going  on  constantly 
during  the  activity  of  the  plant  and  its  rapidity  is  proportional 
to  the  activity  of  the  processes  going  on  in  the  leaves  and  buds. 

126 


PLANT  PHYSIOLOGY  127 

The  method  by  which  the  sap  is  carried  up  the  stem  is  only 
partially  understood;  there  are  several  factors  concerned.  One 
factor  is  osmosis.  The  water  from  the  soil  is  absorbed  by  the 
root  hairs  principally  through  the  physical  force  of  osmosis,  a 
force  which  is  capable  of  causing  some  substances  to  pass,  even 
against  resistance,  through  the  thin-walled  root  hairs,  while 
others  are  rejected.  An  osmotic  pressure  is  thus  produced  in 
the  root,  due  to  the  absorption  of  liquids  from  the  soil,  and  this 
forces  a  current  up  through'  the  stem. 

A  second  factor  is  the  absorptive  power  of  protoplasm.  Liv- 
ing protoplasm  has  a  strong  avidity  for  water  and  absorbs  it 
until  it  is  saturated.  If  a  plant  were  in  absolute  equilibrium, 
each  bit  of  protoplasm  would  absorb  all  the  water  that  it  could 
obtain  and  a  condition  of  rest  would  soon  appear.  If,  however, 
a  cell  loses  any  of  its  liquid,  it  will  have  at  once  a  stronger 
demand  for  water  than  before,  and  will  tend  to  draw  it  away 
from  neighboring  cells  that  are  more  nearly  saturated.  Hence 
in  a  plant  there  will  be  a  constant  flow  of  water  from  saturated 
parts  to  those  less  saturated.  In  an  ordinary  green  plant  there 
are  several  processes  that  use  up  the  water,  all  of  them  especially 
active  in  the  leaves  and  growing  buds  at  the  top  of  the  plant. 
These  are  as  follows: — 

1.  Water  is  being  used  in  the  leaves  to  manufacture  starch. 

2.  New  protoplasm  is  being  made  in  the  leaves  and  in  the 
growing  buds,  and  this  new  protoplasm  demands  water. 

3.  Water  constantly  evaporates  from  the  leaves  through  the 
stomata  (transpiration).    The  extent  of  this  evaporation  varies 
greatly  with  the  warmth  and  dryness  of  the  air  and  also  with 
the  extent  to  which  the  stomata  are  opened.     When  there  is 
abundance  of  water  in  the  plant,  the  stomata  are  widely  open 
and  evaporation  is  rapid;  but  when  the  water  is  insufficient 
these  pores  partly  close  and  evaporation  is  checked.    On  a  warm 
day  when  the  air  is  dry  the  evaporation  is  increased,  but  in  a 
cool  damp  atmosphere  it  is  lessened. 

A  third  factor  is  capillarity;  this  is  the  same  force  that 


128  BIOLOGY 

causes  oil  to  rise  in  the  wick  of  a  lamp.  To  what  extent  this 
contributes  to  the  flow  of  sap  is  uncertain. 

These  factors  combine  to  produce  a  lack  of  water  at  the 
top,  and  an  excess  in  the  roots,  which  produces  a  conse- 
quent tendency  of  the  liquids  in  the  plant  to  flow  upward; 
the  total  result  being  a  flow  of  the  liquids  from  soil  to  root,  from 
root  to  stem,  and  through  the  stem  to  the  leaf  and  bud.  The 
rapidity  of  this  ascent  of  sap  is  directly  proportional  to  the  ac- 
tivity in  the  leaves  and  buds,  since  this  determines  the  extent 
to  which  the  water  is  used  up.  In  warm  bright  sunshine  the 
life  processes  in  the  leaves  are  vigorous,  the  stomata  open,  and 
the  sap  rises  rapidly.  At  night  the  current  is  decreased,  and  in 
winter  the  processes  practically  cease,  to  be  revived  again 
when  the  warm  sun  of  spring  makes  it  possible  for  the  cells  in 
the  leaves  and  buds  to  resume  their  activity.  It  is  known  that 
the  water  rises  chiefly  in  the  large  ducts  of  the  fibrovascular 
bundles,  the  spiral  and  ringed  ducts  serving  for  this  purpose. 
It  does  not  flow,  however,  in  the  cavities  of  these  ducts,  but 
rather  in  their  walls,  passing  from  cell  to  cell  within  the  thick, 
but  evidently  porous,  walls. 

While  these  factors  partly  account  for  the  rise  of  sap,  they  do 
not  explain  the  actual  force  which  lifts  the  water,  rising  as  it 
does  to  the  tops  of  the  tallest  trees.  This  is  difficult  to  explain. 
It  is  generally  thought  to-day  that  the  three  forces  above  men- 
tioned are  sufficient  for  the  process:  (1)  Osmosis:  this  forces 
the  water  from  the  soil,  through  the  root  hairs  into  the  roots, 
and  probably  from  cell  to  cell  within  the  plant,  up  through  the 
root  and  stem  to  the  top  of  the  plant.  (2)  Capillarity:  this 
force  causes  liquids  to  rise  inside  of  small  spaces,  and 
must  play  some  part  in  the  rise  of  water  in  the  plant. 
(3)  Avidity  for  water:  the  demand  for  water  of  the  protoplasm 
at  the  top  of  the  plant,  above  explained,  is  doubtless  an 
active  agent  also  in  producing  the  flow  of  water  from  cell  to 
cell  up  the  plant.  Whether  these  forces  are  sufficient  to  explain 
the  ascent  of  sap  we  do  not  know;  but  at  all  events  the  plant 


PLANT  PHYSIOLOGY  129 

possesses  no  distinct  circulatory  organs,  and  it  is  believed  that 
tnese  physical  forces  are  sufficient  to  account  for  the  lifting  of 
water  from  the  soil  to  the  leaves  and  buds. 

Transfer  of  Substances  Downward.  —  It  is  evident  that  there 
must  be  a  transfer  of  material  downward  as  well  as  an  ascent 
of  sap.  As  we  shall  presently  notice,  plants  are  engaged  in 
making  starch  in  their  leaves,  and  this  starch  is  certainly  carried 
to  all  parts  of  the  plant,  since  it  may  be  stored  in  the  under- 
ground parts.  The  starch  in  a  potato,  for  example,  is  made  in 
the  leaves  and  hence  it  must  be  carried  downward.  The  method 
by  which  the  material  is  carried  from  the  leaves  downward  is 
even  less  understood  than  the  ascent  of  sap,  although  osmosis 
is  undoubtedly  one  of  the  factors.  It  is  known,  however,  that 
the  starch  is  first  changed  to  sugar  and  then  dissolved  in  the 
liquids  of  the  plant.  It  is  also  known  that  these  materials  then 
descend,  not  in  the  same  cells  in  which  sap  is  ascending,  but 
in  the  large  sieve  cells  of  the  bark  (see  Fig.  46)  ,  which  are  the 
cells  chiefly  concerned  in  the  downward  current.  Since  the  bark 
is  needed  for  this  downward  passage  of  food,  we  see  another 
reason  why  the  cutting  of  the  bark  away  from  the  tree  for  a 
short  distance,  girdling,  will  in  time  kill  the  plant,  since  the  food 
materials  made  in  the  leaves  cannot  then  be  carried  to  the  roots 
and  they  will  die  for  lack  of  nourishment. 

PHOTOSYNTHESIS  OR  STARCH  MANUFACTURE 

By  the  process  just  described,  the  water,  with  the  dissolved 
minerals,  is  brought  to  the  chlorophyll  cells  in  the  leaves.  These 
same  cells  are  also  in  direct  contact  with  carbon  dioxid  which 
is  in  the  air  and  is  brought  into  the  leaf  through  the  stomata. 
The  chlorophyll-containing  cells  have  the  wonderful  power  of 
causing  the  carbon  dioxid  obtained  from  the  air,  and  the  water 
obtained  from  the  soil,  to  combine  with  each  other  chemically 
to  form  a  new  product.  The  transformation  is  represented  by 
the  following  equation  :  — 


(Starch) 


130 


BIOLOGY 


It  must  not  be  understood  that  this  equation  is  an  accurate, 
statement  of  what  occurs,  for  we  do  not  know  the  details  of  the 
building  of  starch  from  carbon  dioxid.  There  is  no  doubt  that 
the  process  is  far  more  complex  than  is.  indicated  by  this  simple 
equation.  The  building  of  CO2  and  H2O  into  starch  is  not  done 
by  a  single  step  as  here  represented,  but  in  all  probability  by 
several  steps.  Moreover,  the  starch  molecule  is  by  no  means  a 
simple  molecule  as  the  formula  C6Hi0O5  indicates,  but  some 
multiple  of  this  formula;  how  high  a  multiple  we  do  not  know, 
but  probably  with  many  times  this  number  of  atoms  in  the 
molecule.  The  above  equation  represents  the  ratio  of  the  atoms 

but  not  their  actual  number.  While 
the  details  of  the  method  by  which  the 
complex  molecule  of  starch  is  formed 
are  not  yet  known  to  us,  we  do  know 
that  the  essential  features  represented 
by  this  equation — namely,  that  C02 
and  H2O  are  combined,  that  starch  is 
manufactured,  and  that  oxygen  is  set 
free — are  in  the  main  correct.  This 
process  is  called  photosynthesis  (Gr. 
photos  =  light  +  synthesis  =  composi- 
tion), and  it  is  the  only  known  method 
by  which  starch  can  be  manufactured, 
chemists  having  hitherto  been  unable 
to  make  it  by  any  artificial  means. 

From  the  above  equation  it  will  be 
seen  that  while  carrying  on  photosyn- 
thesis, a  plant  is  using  up  carbon  dioxid 
and  at  the  same  time  liberating  oxygen 
and  producing  starch.  The  oxygen  is 
liberated  in  the  form  of  a  gas  which 
passes  from  the  plant  into  the  atmos- 
phere. The  liberation  of  oxygen  may  be  easily  demonstrated 
by  placing  some  kind  of  green  water  plant  in  a  dish  of  water 


FIG.  67 

Showing  a  method  of  demon- 
strating that  a  plant  while  grow- 
ing eliminates  oxygen  gas.  The 
plant  is  a  green  water  plant,  and 
the  bubbles  which  arise  from  it 
and  collect  in  the  tube  prove  to 
be  oxygen. 


PLANT  PHYSIOLOGY  131 

and  placing  it  in  the  sunlight.  Minute  bubbles  of  gas  will  soon 
make  their  appearance  on  the  plant,  which  will  rise  through  the 
water  and  pass  off  into  the  air.  If  these  bubbles  are  collected  in 
an  inverted  funnel  (Fig.  67)  and  tested  chemically,  the  gas 
proves  to  be  oxygen.  All  green  plants  liberate  oxygen  when 
growing  in  sunlight,  a  process  that  is  exactly  the  reverse  of  the 
respiration  of  animals,  which  absorb  oxygen  gas  and  liberate 
carbon  dioxid  gas. 

Photosynthesis  is  the  foundation  of  all  life,  since  the  life  of 
all  animals  as  well  as  plants  depends  upon  starch.  Its  relations 
to  various  external  conditions  are  as  follows :  — 

Chlorophyll. — Photosynthesis  is  dependent  upon  chlorophyll 
and  hence  occurs  in  green  plants  only.  Moreover,  in  these 
plants,  photosynthesis  occurs  only  in  those  cells  that  contain 
chlorophyll,  and  thus  chiefly  in  the  palisade  and  mesophyll  cells 
of  the  leaf,  although  it  may  take  place  in  other  cells  if  they 
contain  chlorophyll. 

Sunlight. — Photosynthesis  is  dependent  upon  sunlight  and 
therefore  never  occurs  in  plants  unless  they  are  in  the  light. 
The  vigor  of  the  process  is  dependent  also  upon  the  intensity 
of  the  sunlight.  It  is  most  active  in  direct  sunlight,  less  so  in 
diffused  daylight,  and  stops  entirely  when  light  is  withdrawn. 

Carbon  Dioxid. — Photosynthesis  is  dependent  upon  the 
presence  of  carbon  dioxid.  Those  plants  which  live  in  the  air 
will  always  have  plenty  of  carbon  dioxid,  since  the  air  contains 
this  gas.  Water  plants  depend  upon  the  gas  dissolved  in  water. 
The  dependence  of  photosynthesis  upon  carbon  dioxid  can  be 
shown  if  a  green  water  plant  is  placed  in  sunlight  in  ordinary 
water,  when  bubbles  of  gas  (oxygen)  arise  from  it,  showing  the 
presence  of  photosynthesis.  If,  however,  this  plant  be  placed 
in  a  dish  of  boiled  water  which  has  been  cooled,  the  bubbles  do 
not  arise  from  its  leaves,  showing  that  photosynthesis  does  not 
occur.  Boiling  the  water  drives  off  the  carbon  dioxid  dissolved 
in  it,  and  the  plant,  having  no  carbon  dioxid  at  its  command, 
cannot  carry  on  photosynthesis. 


132  BIOLOGY 

Temperature. — Photosynthesis  is  dependent  upon  tempera- 
ture. Even  though  the  sunlight  be  brilliant,  if  the  temperature 
be  below  freezing  photosynthesis  cannot  go  on.  It  can,  how- 
ever, take  place  in  temperatures  very  slightly  above  freezing, 
and  will  continue  from  this  point  up  to  moderately  high  tem- 
peratures. At  higher  temperatures,  120°  to  130°  F.,  the  process 
stops.  The  temperature  at  which  photosynthesis  goes  on  most 
rapidly,  the  optimum  temperature,  varies  with  different  plants, 
depending  upon  the  structure  of  the  plant  itself.  Some  plants 
are  so  constructed  that  they  can  grow  only  at  moderately  low 
temperatures,  and  others  only  at  high  temperatures.  In  some 
of  the  arctic  plants,  photosynthesis,  as  well  as  all  the  other 
functions  of  the  plant,  goes  on  very  readily  when  the  tempera- 
ture is  not  much  above  freezing,  whereas  in  tropical  plants 
photosynthesis  does  not  occur  unless  the  temperature  is  high. 

METASTASIS 

Photosynthesis  may  be  spoken  of  as  food  manufacture,  for 
the  starch  thus  made  is  later  utilized  for  the  life  processes  of 
the  plant.  The  use  of  this  starch  as  food  is  generally  spoken  of 
under  the  term  metastasis  (Gr.  meta  =  beyond  +  histanai  = 
to  place).  This  is  too  complicated  a  process  to  be  described 
here  in  detail,  and  only  a  few  of  the  main  features  will  be  briefly 
explained. 

As  already  stated,  the  plants  take  in  through  their  root  hairs 
not  only  water  but  a  number  of  ingredients  dissolved  in  it. 
Among  these  are  nitrates,  phosphates,  potash,  and  various  other 
substances  in  smaller  quantities.  All  of  these  substances  are 
carried  up  through  the  plant  and  distributed  so  that  each  living 
cell  may  receive  some  of  this  dissolved  material.  The  starch, 
formed  chiefly  in  the  leaves,  as  we  have  seen  is  converted  into 
sugar,  chiefly  in  the  night,  and  then  transported  through  the 
plant  in  the  sieve  cells  of  the  bark.  The  living  cells  in  the  various 
parts  then  take  the  water  and  minerals  brought  with  the  ascend- 
ing sap,  and  the  sugars  brought  from  the  leaves,  and  by  changes 


PLANT  PHYSIOLOGY  133 

of  complex  but  unknown  nature  cause  them  to  combine  within 
the  cell  protoplasm  into  new  substances. 

These  new  substances  are  of  many  varieties.  The  most  im- 
portant among  them  is  the  class  of  compounds  which  we  have 
already  learned  to  call  proteids.  Proteids  contain  chiefly  the 
elements  carbon,  oxygen,  hydrogen,  and  nitrogen,  and  are  built 
out  of  the  nitrates  and  other  minerals  absorbed  from  the  soil, 
in  combination  with  the  sugars  brought  to  them  from  the  leaves. 
Proteids  are  not  the  only  substances  manufactured  in  the  plant 
cells.  Fats  are  produced  which  may  be  stored  away  in  the  plants 
or  used  for  other  purposes.  Wood  is  also  made  and  deposited 
around  the  protoplasm,  forming  the  walls  of  the  wood  cells. 
Numerous  other  substances  are  produced  which  we  need  not 
mention,  for  the  end  result  is  the  growth  of  all  parts  of  the 
plant  which  increases  in  size  as  these  new  substances  are  formed. 
In  all  cases,  however,  the  starch  made  by  the  leaves  is  the  foun- 
dation of  the  new  substances  made.  Starch  is  always  used  up 
and  the  plant  can  grow  only  so  long  as  it  has  starch  at  hand  in 
abundance.  This  process  of  using  starch  and  making  other 
substances  is  known  as  metastasis. 

One  of  the  results  of  the  use  of  starch  for  any  of  these  purposes 
is  a  combination  of  part  of  its  carbon  with  oxygen,  forming  CO2. 
This  is  a  process  similar  to  the  respiration  of  animals,  and  the 
CC>2  is  in  plants,  as  in  animals,  a  waste  product  which  must  be 
excreted.  It  is  thus  seen  that  plants  carry  on  two  opposite 
processes.  By  photosynthesis  CO2  is  utilized,  starch  is  formed 
and  0  is  set  free;  by  metastasis  O  is  used,  starch  is  destroyed  and 
C02  is  set  free.  During  the  ordinary  life  of  a  plant  in  daylight, 
although  both  processes  are  going  on  simultaneously,  photo- 
synthesis is  much  more  vigorous  than  metastasis,  and  much 
more  starch  is  made  by  the  plant  than  is  used,  so  that  oxygen 
is  constantly  eliminated.  Photosynthesis,  since  it  takes  place 
only  in  sunlight,  can  occur  only  in  the  daytime,  while  metastasis, 
requiring  no  sunlight,  can  go  on  in  the  night.  The  process  of 
metastasis  goes  on  fully  as  well,  and  certain  phases  go  on  better, 


134  BIOLOGY 

in  the  darkness  than  in  the  light.  As  a  result,  green  plants  in 
sunlight  and  in  the  daytime  give  off  a  surplus  of  oxygen,  while 
in  the  night  they  are  giving  off  carbon  dioxid  but  no  oxygen. 
Oxygen  gas  is  a  material  that  is  utilized  by  animal  life,  while 
carbon  dioxid  gas  is  a  waste  product  of  animals  as  well  as  plants. 
Hence  it  has  been  said  that,  in  the  daytime  plants  are  useful  in 
a  living  room,  while  in  the  night-time  they  are  harmful.  There 
is  really  no  foundation  for  this  claim,  since  the  amount  of  carbon 
dioxid  given  off  by  a  few  plants  in  a  room  is  so  slight  that  it  is 
of  no  practical  significance  in  its  bearing  upon  animal  life.  In 
nature,  however,  the  plant  and  animal  life  balance  each  other; 
while  animals  absorb  the  oxygen  given  off  by  plants,  they  them- 
selves give  off  carbon  dioxid  that  is  utilized  by  plants;  and  thus 
the  condition  of  the  atmosphere  is  kept  practically  constant 
so  far  as  concerns  its  content  of  both  oxygen  and  carbon  dioxid. 

In  general,  plants  manufacture  far  more  starch  than  they 
need  for  their  own  life.  The  surplus  is  stored  in  some  form  as 
starch,  sugar,  fat,  proteid,  or  some  other  material,  and  upon  this 
surplus  the  whole  animal  world  is  nourished. 

All  ordinary  green  plants  carry  on  this  process  of  photosyn- 
thesis. Fungi,  illustrated  by  bacteria,  yeasts,  molds,  mushrooms, 
etc.  (Figs.  32,  34,  42),  all  agree  in  lacking  the  green  chlorophyll 
and  are  for  this  reason  sometimes  called  colorless  plants.  Since 
they  have  no  chlorophyll  they  are  unable  to  carry  on  the  process 
of  photosynthesis,  unable  to  utilize  the  energy  of  sunlight  and 
manufacture  starch.  But  they  must  have  energy  as  well  as 
green  plants  for  their  life,  and  are  therefore  dependent  upon  the 
latter  for  their  food.  The  Fungi  are  commonly  found  growing 
and  feeding  upon  organic  foods,  and  are  quite  unable  to  utilize 
the  minerals  of  the  soil  and  the  gases  of  the  air.  They  are 
usually  found,  therefore,  in  the  midst  of  masses  of  decaying 
organic  refuse,  on  dead  tree  trunks,  in  manure  heaps,  growing 
from  rotting  leaves,  etc.  They  feed  upon  the  remains  of  past 
generations  of  green  plants,  having,  as  we  shall  see  later,  a  very 
important  part  to  play  in  nature's  food  cycle. 


PLANT  PHYSIOLOGY 


135 


PHOTOSYNTHESIS  AND  METASTASIS  CONTRASTED 

The  relation  between  these  two  functions  of  plant  life  may 
be  better  understood  by  the  following  contrast: — 


PHOTOSYNTHESIS 
Takes  place  only  in  green  cells. 
Takes  place  only  in  light. 


C02  is  absorbed  and  used  up 
and  oxygen  given  off. 

Carbohydrates  are  formed. 
The  plants  grow  in  weight. 

The  energy  of  sunlight  is  stored ; 
see  Chapter  XV. 


METASTASIS 
Takes  place  in  all  living  cells. 

Takes    place    equally    well    in 
darkness. 

Oxygen  is  absorbed  and  used 
and  CO2  given  off. 

Carbohydrates  are  destroyed. 

The  plants  lose  weight,  but  may 
increase  in  size. 

The  stored  energy  of  sunlight  is 
liberated  and  used. 


The  forces  concerned  in  starch  making  and  the  building  of 
proteids  and  other  materials  are  ordinary  chemical  and  physical 
forces.  While  we  cannot  cause  these  particular  chemical  com- 
binations to  occur  in  our  laboratories,  and  do  not  understand 
them  fully,  we  do  know  enough  about  them  to  prove  that  they 
belong  to  the  ordinary  forces  of  chemical  affinity.  In  starch 
making  the  atoms  are  combined  in  ordinary  proportions,  and 
there  is  no  reason  for  thinking  that  any  other  factors  are  con- 
cerned besides  those  of  chemical  affinity. 

MISCELLANEOUS  FUNCTIONS  OF  PLANT  LIFE 

Besides  the  processes  of  photosynthesis  and  metastasis,  the 
only  other  prominent  function  of  plant  life  is  reproduction. 
The  two  functions  of  motion  and  coordination,  which  are  very 
prominent  in  animal  forms,  are  very  slightly  developed  among 
plants. 


136 


BIOLOGY 


Motion. —  The  most  striking  distinction  ordinarily  recog- 
nized between  animals  and  plants  is  the  absence  of  the  power 
of  motion  in  plants  and  its  presence  in  animals.  This  distinc- 
tion, however,  is  by  no  means  a  sharp  one,  for  motion  is  not 
wholly  lacking  in  plants.  Many  of  the  lower  types  of  plants 
are  capable  of  locomotion.  This  is  confined  largely  to  the  micro- 
scopic forms,  and  in  some  plants  it  is  present  only  in  their  re- 
productive spores.  For  example,  Ulothrix  (see  page  93)  is  a 
motionless  organism  in  its  ordinary  adult  form,  but  produces 
reproductive  spores,  called  zoospores,  which  swim  rapidly  in  the 

water.  Among  other 
microscopic  plants, 
locomotive  power  is 
found  even  in  the  adult 
life  of  the  animal. 
This  is  true  of  Osdl- 
laria,  Diatoms,  and 
some  other  organisms ; 
Fig.  68.  Among  the 
higher  plants  no  active  type  of  locomotion  is  found,  although 
many  of  them  are  constructed  in  such  a  way  that  they  may  be 
carried  to  and  fro  by  motile  animals.  Even  among  the  highest 
plants,  however,  a  certain  amount  of  motion  is  developed  in  the 
different  parts  of  the  plant.  Among  the  flowers  of  the  highest 
groups  of  plants,  motion  is  developed  in  certain  parts  ot  the 
flowers  for  the  distribution  of  pollen.  In  most  of  the  highest 
class  also,  careful  study  has  shown  that  the  leaves  are  constantly 
in  a  state  of  slow  motion,  waving  to  and  fro  during  the  growth 
of  the  plant  in  sunlight.  Of  course  the  leaves  are  almost  always 
moved  by  the  wind,  but  quite  independently  of  air  currents  they 
have  a  motion  of  their  own  which  can  be  detected  by  a  careful 
recording  apparatus.  It  is  thought  that  this  motion  is  due 
principally,  perhaps  entirely,  to  the  unequal  evaporation  of 
water  on  different  sides  of  the  stem.  At  all  events  it  is  so  slight 
that  it  can  hardly  be  considered  true  motion,  and  it  certainly 


FIG.  68. —  THREE  PLANTS  HAVING 

THE  POWER   OF   MOTION 

A  and  B,  Diatoms,  which  move  readily  through  water; 
C,  Osdllaria,  which  simply  waves  back  and  forth. 


PLANT  PHYSIOLOGY  137 

is  not  locomotion.  In  addition  to  this,  some  plants  have  the 
peculiar  property  of  closing  their  leaves  in  the  night.  The  leaves 
droop  and  close  themselves  in  such  a  way  as  to  present  a  small 
surface  for  evaporation.  This  motion  is  sometimes  spoken  of 
as  the  sleep  of  plants.  It  is  not  developed  in  all,  but  it  is  more 
common  than  has  generally  been  believed. 

Thus,  while  it  is  believed  that  plants  do  not  as  a  rule  possess 
the  power  of  motion  and,  except  in  the  lowest  forms,  no  power 
of  locomotion,  it  is  not  absolutely  true  that  motion  is  lacking 
in  the  vegetable  kingdom.  Speaking  in  general,  however,  plants 
are  characterized  by  absence  of  motility. 

Coordinating  Functions. — Plants  have  nothing  whatever  that 
corresponds  to  a  nervous  system  in  the  sense  of  possessing  nerves 
or  nerve  fibers  which  coordinate  the  different  parts  of  the  body. 
There  is  practically  no  coordination  between  the  functions  carried 
on  in  the  different  parts  of  the  plant.  True  sensory  functions  are 
also  lacking  from  plants.  In  a  general  way  the  protoplasm  of 
plants,  as  well  as  that  of  animals,  is  sensitive.  All  protoplasm 
reacts  under  certain  stimuli  and  is  therefore  sensitive.  Moreover, 
there  are  some  of  the  higher  plants  which  react  so  quickly  and  so 
strongly  to  certain  stimuli  that  they  are  spoken  of  as  sensitive 
plants.  In  the  common  so-called  sensitive  plant  a  touch  upon 
the  leaf  will  cause  the  leaf  to  close,  and  a  slight  touch  of  the 
branch  will  cause  all  the  leaves  on  that  branch  to  droop.  Such  a 
condition,  however,  is  very  unusual  among  plants,  and  in  these 
cases  it  is  incorrect  to  speak  of  the  plants  as  sensitive  in  any 
proper" sense.  There  is  no  reason  for  thinking  that  the  plant  has 
any  sensation,  i.e.,  any  true  consciousness;  and  all  that  is  meant 
by  being  sensitive  in  these  cases  is  a  quick  ability  to  respond  to 
an  external  stimulus. 


CHAPTER   VII 

MULTICELLULAR   ANIMALS:    HYDRA   FUSCA 
GENERAL  LIFE  FUNCTIONS  OF  ANIMALS 

THE  life  of  animals  is  much  more  complicated  than  that  of 
plants  and  the  animal  body  is  correspondingly  more  complex. 
It  will  make  the  study  of  multicellular  animals  more  intelligible 
if  at  the  outset  we  notice  certain  general  functions  of  life  that 
are  exhibited  by  all  higher  animals.  They  are  as  follows:  — 

Alimentation  (Lat.  alimentum  =  food).  — The  process  of 
food  getting  is  called  alimentation.  The  organs  concerned  in 
it  are  those  that  take  food  into  the  body,  those  that  digest  it, 
and  finally  those  that  absorb  it  into  the  circulating  medium. 

Circulation. — The  process  by  which  food  and  other  ingredients 
are  transported  through  the  body  is  called  circulation.  Usually 
it  is  brought  about  by  a  circulating  medium  called  the  blood,  by 
a  series  of  tubes  in  which  the  blood  is  carried,  known  as  blood 
vessels,  and  by  a  pump,  or  heart,  designed  to  keep  the  blood  in 
motion.  In  some  of  the  smaller  animals  this  system  of  organs 
is  far  simpler,  neither  blood  vessels  nor  a  heart  being  present; 
but  some  form  of  circulation  is  always  found. 

Respiration. — The  chief  chemical  process  in  the  animal  body 
is  oxidation,  i.e.,  the  combination  of  the  food  with  the  oxygen. 
For  this  purpose,  oxygen  gas  must  be  absorbed  by  the  blood. 
As  a  result  of  the  oxidation  of  the  food  another  gas  (CO2).  arises, 
which  is  also  taken  up  by  the  blood  and  must  be  eliminated, 
since  it  is  a  waste  product.  The  function  by  which  these  two 
gases  (O  and  CO2)  are  absorbed  and  discharged  is  called  respira- 
tion. Respiration  is  thus  a  gas  exchange  that  takes  place  be- 
tween the  body  and  the  surrounding  medium. 

Metabolism  (Gr.  meta  =  beyond  +  ballein  =  to  throw). — 
The  foods  taken  into  the  body  are  eventually  combined  with 
the  oxygen  taken  in  by  respiration  and  as  a  result  new  products 

138 


HYDRA  FUSCA  139 

arise,  some  of  which  are  useful,  while  others  are  waste  products. 
The  result  of  the  combination  of  food  with  oxygen  is,  that  a 
certain  amount  of  force  is  liberated  in  the  same  way  that  heat 
is  liberated  from  coal  when  it  is  burned.  This  force  varies 
according  to  the  amount  of  activity  of  the  animal  life.  The 
whole  process  of  chemical  change  by  which  the  food  is  used  is 
called  metabolism.  Two  distinct  phases  of  it  may  be  recognized : 
anabolism  (Gr.  ana  =  up) ,  the  process  by  which  complex  sub- 
stances are  built  out  of  simpler  ones;  and  katabolism  (Gr.  kata  = 
down),  the  process  by  which  complex  substances  are  torn  down 
into  simpler  ones.  In  animals  the  latter  are  more  extensive  than 
the  former. 

Excretion. — The  function  of  getting  rid  of  the  waste  products 
of  metabolism  is  called  excretion.  These  products  are  no  longer 
valuable  but  act  as  a  direct  poison  to  the  body  if  allowed  to 
remain.  These  waste  products  are  solid,  liquid,  or  gaseous. 
The  gases  are  excreted  by  respiration,  as  just  described.  In 
higher  animals  the  liquids  are  carried  off  by  the  lungs,  by  the 
skin,  and  by  special  organs  called  kidneys.  It  must  be  remem- 
bered that  excretion  does  not  refer  to  the  passage  from  the 
intestines  of  the  undigested  food.  This  undigested  food  has 
never  become  part  of  the  body  and  its  passage  from  the  intes- 
tines is  not  strictly  excretion.  There  is  apt  to  be  confusion  in 
the  use  of  the  terms,  as  the  undigested  food  which  passes  through 
the  intestines  frequently  goes  by  the  name  of  excreta.  In  the 
strict  sense,  however,  the  excreta  or  faeces  are  not  excretions. 

Motion. — Practically  all  animals  possess  some  power  of  mo- 
tion and  have  special  organs  adapted  for  bringing  it  about. 

Support. — The  living  parts  of  an  animal  (protoplasm)  are 
made  up  of  a  soft,  jelly-like  substance,  too  non-resistant  to 
have  the  power  to  hold  any  particular  shape.  If  the  animal  is 
small  the  resisting  power  of  the  jelly  may  be  sufficient  to  preserve 
its  shape;  but  in  large  animals  it  is  necessary  to  have  some  hard 
support  for  holding  the  soft  parts.  This  hard  supporting  sub- 
stance may  be  in  the  form  of  a  skeleton  or  shell. 


140  BIOLOGY 

Coordination. — The  numerous  activities  of  the  animal  body 
are  brought  into  harmonious  action  for  a  common  purpose. 
The  function  by  which  they  are  related  to  one  another  is  known 
as  coordination  (Lat.  con  =  together  +  ordinare  =  to  regu- 
late), and  the  system  of  organs  that  produces  this  coordination 
is  generally  spoken  of  under  the  name  of  the  nervous  system. 

Reproduction. — This  is  the  function  of  producing  new  individ- 
uals like  the  old,  which  prevents  the  species  from  disappearing 
from  the  earth. 

The  nine  functions  thus  outlined  are  necessary  to  the  life  of 
all  animals.  In  a  few  of  the  lower  animals,  some  of  these  func- 
tions are  very  slightly  developed;  and  in  quite  a  number  of 
smaller  animals  we  do  not  find  any  special  system  of  organs 
devoted  to  some  of  these  functions.  For  example,  many  small 
animals  have  no  skeleton,  and  some  of  the  very  simple  ones 
have  no  organs  that  can  properly  be  called  a  coordinating  sys- 
tem, since  all  of  the  functions  of  the  animal  take  place  in  one 
small  cell  where  no  coordination  is  needed.  But  speaking  in 
general,  all  animals,  high  or  low,  carry  on  all  these  functions. 

ANIMAL  BIOLOGY 

In  our  consideration  of  animal  Biology  we  shall  study  three 
animals,  chosen  to  illustrate  different  grades  of  structure. 
Hydra  will  be  an  example  of  one  of  the  simplest  multicellular 
animals;  the  earthworm,  an  animal  of  moderate  complexity; 
and  the  frog  will  be  an  example  of  the  more  highly  complex 
types. 

HYDRA   FUSCA:   A   SIMPLE   MULTICELLULAR  ANIMAL 

General  Description. — The  brown  Hydra  is  a  very  common 
water  animal  and  may  be  found  in  almost  any  pond  on  the  under 
side  of  lily  pads  or  pond  weeds.  Here  it  may  be  seen  as  a  small 
reddish  body,  just  large  enough  to  be  visible.  Our  common 
Hydra  ( Hydra  fused)  is  of  a  brown  color,  but  another  common 
species  (Hydra  viridis}  is  bright  green.  If  the  animal,  still 


HYDRA  FUSCA  141 

attached  to  the  lily  leaf,  be  removed  from  the  pond,  placed  in 
a  dish  of  water  and  left  undisturbed  for  a  time,  it  will  slowly  ex- 
pand and  assume  the  form  represented  in  Figure  69  A.  It  shows 
then  a  slender  body  about  a  quarter  of  an  inch  or  less  in  length, 
attached  at  one  end  to  some  other  solid  object.  At  the  other 
end  it  bears  a  crown  of  tentacles,  which  in  the  brown  Hydra 
are  from  five  to  ten  in  number,  and  in  the  green  Hydra  are  from 
five  to  twelve.  These  tentacles  are  very  delicate,  hairlike  bodies, 
which  may  be  expanded  to  considerable  length,  as  at  A,  but  when 
contracted,  shrink  into  minute  knobs  hardly  big  enough  to  be 
seen.  Indeed,  the  whole  body  of  the  Hydra  is  extremely  con- 
tractile, and  though  when  undisturbed  it  may  be  a  half  an  inch 
or  more  in  length,  on  being  disturbed  it  will  contract  into  a 
small  body  no  larger  than  a  pinhead;  see  Fig.  69  B.  Hydra 
seems  at  first  to  be  a  stationary  animal,  although  it  can  move 
its  tentacles  slowly  to  and  fro  in  the  water.  A  careful  examina- 
tion, however,  shows  that  it  has  some  power  of  motion;  the  ani- 
mal, creeping  by  means  of  its  base,  can  move  slowly  over  the 
object  upon  which  it  is  fastened.  Occasionally  also  it  moves 
by  turning  end  over  end.  It  first  attaches  its  tentacles  to  the 
object  to  which  its  base  is  attached.  Then  the  base  lets  go  its 
hold  and  is  moved  over  and  fastened  again  in  another  spot. 
The  tentacles  let  go  their  hold  and  the  animal  straightens  up. 
The  movement  is  not  unlike  that  of  a  boy  turning  a  handspring. 

Structure. —  In  the  midst  of  the  crown  of  tentacles  is  a  little 
conical  projection,  on  the  top  of  which  is  a  mouth.  This  is 
star-shaped  rather  'than  circular,  and  opens  into  a  cavity  which 
fills  the  whole  of  the  body  of  the  Hydra  and  even  extends  into 
its  tentacles.  This  cavity  is  the  digestive  cavity  and  is  called 
the  gastrovascular  cavity;  see  Fig.  C. 

Hydra  is  a  true  multicellular  animal,  made  up  of  many  thou- 
sands of  cells  which  are  not  alike  but  show  a  considerable  differ- 
entiation and  have  a  division  of  labor  among  them.  All  of  these 
cells,  however,  are  arranged  into  two  layers,  one  on  the  out- 
side called  the  ectoderm  (Gr.  ectos  =  outside  +  derma  =  skin),  ec 


14? 


BIOLOGY 


FIG.  69. —  HYDRA 

A,  an  animal  in  its  expanded  form;  B,  the  same  animal  contracted;  C,  a  diagram  of  the 
longitudinal  section  of  the  animal,  showing  the  internal  structure;  D,  an  epithelio-muscle 
cell;  E,  a  bit  of  the  body  wall  highly  magnified  showing  the  two  layers  of  the  body;  F,  a 
digestive  cell;  G,  one  of  the  nematocysts  with  its  thread  extruded;  H,  a  second  type  of 
nematocyst  with  the  coiled  thread  within  the  sac;  /,  nematocyst  of  the  third  type  with  its 
thread  extruded;  J,  a  bit  of  the  tentacle,  very  highly  magnified,  showing  the  batteries  of  the 
nematocysts;  K,  two  of  the  secreting  cells  of  the  basal  disk. 


en,  cnidocil; 
ec,  ectoderm; 
en,  endoderm; 


m,  mouth; 
mes,  inesogloea; 


o,  ovary; 
s,  spermary. 


HYDRA  FUSCA  143 

(Fig.  C),  and  one  on  the  inside  called  the  endoderm  (Gr, 
endon  =  inside),  en.  These  two  layers  are  found  throughout  the 
body,  both  the  ectoderm  and  endoderm  extending  into  the 
tentacles  to  their  very  tips.  Hydra  is  thus  a  double  sac  with 
no  space  between  its  two  layers.  The  layers  of  ectoderm  and 
endoderm  are  not  in  actual  contact  with  each  other,  but  are- 
separated  by  a  thin  supporting  layer  known  as  a  mesogloea 
(Gr.  mesos  =  middle  +  gloia  =  glue),  mes.  By  means  of  this 
intermediate  layer,  the  ectoderm  and  endoderm  are  very  firmly 
attached  to  form  one  solid  mass,  forming  a  body  wall  made  up 
of  two  layers  of  cells. 

Ectoderm. — The  ectoderm  is  made  of  two  chief  kinds  of  cells. 
The  first  of  these  is  the  epithelio-muscle  cells;  Fig.  D.  These 
are  in  the  shape  of  cones,  with  their  broad  ends  outward  and 
their  tapering  ends  toward  the  mesoglcea.  At  the  tapering  ends 
some  long  fibers  protrude  which  extend  over  the  body  of  the 
animal  next  to  the  mesogloea.  The  great  contractility  of  Hydra 
is  due  to  these  fibers.  The  second  type  of  cells  is  the  interstitial 
(Lat.  inter  =  between  +  sistere  =  to  stand)  cells.  These  are 
found  between  the  first  cells  and  are  somewhat  smaller  than  the 
epithelio-muscle  cells.  They  are  chiefly  interesting  because 
they  produce  a  very  peculiar  type  of  organ  possessed  by  Hydra 
known  as  the  nematocysts  (Gr.  nema  =  thread  +  cystis  =  sac) . 
The  nematocysts,  or  stinging  cells,  are  little  sacs  scattered  all 
over  the  outside  of  the  body  of  the  animal,  especially  in  the 
tentacles.  Each  of  these  is  an  oval  sac,  one  side  of  which  is 
pushed  inward  like  the  finger  of  a  glove  inverted  into  its  palm; 
Figs.  G,  H}  and  7.  This  inverted  portion  is  in  the  form  of  a 
long  thread,  much  longer  than  the  diameter  of  the  sac,  and  is 
wound  up  in  a  long  coil  inside  of  it;  Fig.  H.  Besides  this 
thread  the  sac  contains  a  liquid.  The  peculiarity  of  these 
cells  is  that  under  a  proper  stimulus  the  minute  thread  may  be 
inverted  from  the  sac  as  shown  in  Figure  G.  This  inverted  por- 
tion when  discharged  carries  with  it  a  small  quantity  of  poison, 
and  thus  each  thread  serves  as  a  little  poison  dart.  The  thread 


144  BIOLOGY 

is  not  shot  away  from  the  animal,  but  only  protruded  to  its 
length.  If  any  small  animal  with  thin  skin  comes  in  contact 
with  the  Hydra,  some  of  these  threads  are  discharged,  the 
animal  is  hit  by  them,  paralyzed  by  the  poison,  and  then 
transferred  to  the  Hydra's  mouth  by  means  of  its  tentacles. 
In  Hydra  these  cells  are  so  small  that  they  cannot  pierce  the 
human  skin  and  their  sting  cannot  be  felt;  in  some  allied  animals, 
like  the  jellyfishes  or  sea  nettles,  these  cells,  although  the  same 
in  structure  as  those  of  the  Hydra,  are  much  larger  and  may  pro- 
duce a  severe  sting.  After  the  thread  is  once  discharged,  it  can- 
not be  withdrawn  again  into  its  sac;  the  cell  thus  becomes  use- 
less. It  is  necessary,  therefore,  for  Hydra  to  be  constantly 
replacing  them,  and  new  nematocysts  are  constantly  growing 
from  the  old  interstitial  cells.  The  special  cell  that  produces 
the  nematocyst  is  known  as  the  cnidoblast.  This  is  simply  one 
of  the  interstitial  cells  which  has  for  its  function  the  production 
of  these  stinging  sacs. 

In  the  brown  Hydra  there  are  three  kinds  of  nematocysts. 
The  larger  one,  G,  is  somewhat  pear-shaped,  and  when  its  thread 
is  protruded  it  has,  close  to  the  base  of  the  thread,  two  or  three 
slender  barbs  projecting  backwards.  When  the  thread  is  dis- 
charged from  the  cell  these  barbs  are  ejected  first.  It  is  thought 
that  their  function  is  to  pierce  the  skin  of  the  animal  into  which 
the  poison  is  to  be  ejected.  Close  to  the  base  of  the  thread  is  a 
minute  little  organ  called  the  cnidocil  (Gr.  cnide  =  thistle)  whose 
function  is  unknown ;  Fig.  G,  en.  It  has  been  supposed  that  it 
helps  to  discharge  the  cell  as  a  trigger  does  a  gun.  This  is  doubt- 
ful, for  it  is  known  that  the  cell  is  most  easily  discharged  by 
changing  the  internal  pressure,  rather  than  by  any  mechanical 
touch  upon  this  cnidocil.  The  second  of  the  nematocysts  in 
the  Hydra,  H,  is  smaller  but  more  elongated.  The  thread  when 
discharged  is  very  different  in  shape,  lacks  the  projecting  barb, 
and,  relative  to  the  size  of  the  sac,  is  much  longer.  The  third 
cell  is  smaller  still,  I,  oval  in  shape,  and  contains  a  thread  that 
when  discharged  always  coils  up  in  a  spiral  form.  It  is  thought 


HYDRA  FUSCA  145 

that  this  spiral  coiling  is  to  enable  the  animal  to  adhere  to  the 
minute  spines  or  hairs  of  its  prey  by  coiling  around  them  in  a 
corkscrew  fashion. 

The  nematocysts  are  scattered  all  over  the  body  of  Hydra 
except  in  its  base.  In  some  parts,  especially  in  the  tentacles, 
they  are  grouped  into  little  bunches  which  project  from  the  side 
and  form  tubercles;  Fig.  J.  These  little  clusters  are  spoken  of 
as  batteries. 

The  Basal  Disk. — The  base  of  Hydra  is  different  from  the 
rest  of  the  body.  It  secretes  a  sticky  substance  by  means  of 
which  the  animal  attaches  itself  to  an  object.  This  base  has 
the  power  of  causing  the  animal  to  glide  very  slowly  over  the 
object  upon  which  it  is  attached,  though  the  exact  method  by 
which  this  motion  is  produced  is  not  known.  In  this  part  of  the 
body  the  nematocysts  are  lacking,  and  the  epithelio-muscle  cells 
not  only  have  muscle  fibers  but  some  of  them  have  the  func- 
tion of  secreting  a  cement,  and  differ  in  appearance  from  those 
of  the  rest  of  the  body;  Fig.  K. 

Endoderm. — The  endoderm  is  about  twice  as  thick  as  the 
ectoderm  and  contains  cells  of  two  kinds,  known  as  the  digestive 
cells  and  the  secretory  cells.  The  digestive  cells  are  long  and 
cup-shaped,  and  have,  extending  from  their  base  next  to  the 
mesoglcea,  fibers  of  contractile  substance.  At  their  inner  or 
free  end  they  bear  two  lashing  flagella;  Fig.  F.  It  is  interest- 
ing to  note  that  the  free  end  of  these  cells  may  be  protruded  in 
the  form  of  pseudopodia,  much  like  those  already  seen  in  the 
Amoeba,  and  that  they  are  able  to  take  into  their  bodies  small 
solid  particles  of  food  which  are  then  probably  digested  within  the 
cells  of  the  body  itself.  Thus  Hydra  has  a  function  of  digestion 
similar  to  that  of  the  Amoeba,  being  able,  to  a  certain  extent, 
to  take  inside  of  its  digesting  cells  solid  particles  of  food  and  to 
digest  them  (intracellular  digestion).  The  chief  digestion,  how- 
ever, is  carried  on  by  the  other  cells,  the  secretory  cells.  These 
are  smaller  than  the  digestive  cells  and  lack  the  contractile 
fibers  at  their  base.  They  produce  a  secretion  which  is  discharged 


146  BIOLOGY 

from  their  free  surface  into  the  cavity  of  the  body,  and  is  thus 
poured  upon  the  food  which  is  taken  into  the  mouth  and  lies 
free  in  the  gastrovascular  cavity  (intercellular  digestion). 

Hydra  has  thus,  in  addition  to  a  method  of  digestion  which 
resembles  that  of  the  Amoeba,  the  power  of  producing  a  digestive 
secretion,  which  is  poured  upon  the  food  in  the  general  cavity, 
only  the  nutritious  portions  of  the  food  being  absorbed  after 
digestion.  This  method  of  digestion,  which  is  peculiar  to  the 
higher  animals,  is,  in  the  Hydra,  combined  with  the  simple 
method  of  digestion  characteristic  of  the  PROTOZOA;  and  in 
this  respect  the  Hydra  represents  a  transition  stage  between  the 
unicellular  animals  and  the  higher,  multicellular  forms.  The 
function  of  the  hairlike  flagella  on  the  endodermal  cells  appar- 
ently is  to  keep  in  circulation  the  liquids  present  in  the  body  and 
thus  to  aid  in  bringing  the  digestive  juices  in  contact  with  the 
food  which  lies  in  the  cavity.  This  is  the  only  trace  of  a  circu- 
latory system  that  the  Hydra  possesses. 

Nervous  System. — According  to  recent  investigation,  it  seems 
that  Hydra  possesses  a  very  simple  nervous  system,  so  delicate, 
however,  that  it  requires  special  methods  of  study;  and  very 
little  is  known  about  it.  There  is  a  series  of  nerve  cells  near 
the  mouth  and  another  near  the  base  of  the  animal,  and  these 
are  connected  with  excessively  delicate  fibers  passing  over  the 
body.  There  are  sensory  cells  on  the  surface  layer  that  are 
probably  connected  with  the  nerve  cells,  and  some  of  the  nerve 
cells  apparently  send  nerve  fibers  to  the  contractile  fibers  of  the 
epithelio-muscle  cells.  This  system  is,  however,  very  simple 
and  rudimentary,  and  is  of  interest  chiefly  as  the  simplest  type 
of  nervous  system  found  among  animals. 

Growth  and  Budding.— The  food  of  Hydra  consists  mainly 
of  minute  water  animals  which  are  captured  by  means  of  its 
tentacles.  The  tentacles  are  protruded  into  water,  and  small 
animals,  coming  in  contact  with  them,  are  paralyzed  by  the 
discharge  of  the  nematocysts.  The  tentacles  then  transfer  the 
food  to  the  mouth.  It  is  pushed  into  the  gastrovascular  cavity 


HYDRA  FUSCA  147 

and  then,  by  the  contraction  of  the  body  wall,  forced  downward 
to  the  basal  end  of  the  cavity.  Here  it  is  mixed  with  the  diges- 
tive juices  of  the  animals  and  slowly  digested.  In  time  the  diges- 
tive parts  are  dissolved  and  absorbed  into  the  cells  that  form 
the  body  wall  and  are  assimilated.  After  all  the  nutritious  por- 
tions have  been  digested  and  absorbed  from  the  food  particles, 
the  undigested  refuse  is  then  ejected  from  the  mouth  by  a  sudden 
contraction  of  the  body  and  opening  of  the  mouth,  which  throws 
the  ejected  portions  some  distance  from  the  animal.  As  the 
result  of  digestion  and  assimilation,  the  animal  grows. 

After  it  reaches  a  certain  size,  rarely  more  than  one-half  an 
inch  in  length,  the  further  growth  shows  itself  in  the  formation 
of  buds  which  appear  on  the  sides  of  the  old  individual;  see 
Fig.  A.  These  buds  rapidly  increase  in  length,  and  after  a 
time  a  circle  of  minute  secondary  buds  can  be  seen  at  their  tips. 
These  secondary  buds  are  rudimentary  tentacles,  for  they  in- 
crease in  length  till  eventually  they  become  new  tentacles.  In 
the  middle  of  the  circle  of  tentacles  thus  formed  a  small  opening 
makes  its  appearance,  which  forms  a  new  mouth  at  the  end  of  the 
growing  bud.  After  a  time  the  bud  itself  separates  from  the  body 
of  the  animal  from  which  it  grew  and  floats  off  by  itself  as  an 
independent  individual,  identical  in  structure  with  the  one  from 
which  it  came,  though  somewhat  smaller.  In  this  way  the 
Hydra  reproduces  itself  indefinitely  by  budding  (gemmation)  as 
long  as  it  has  sufficient  food  and  proper  conditions  for  feeding 
and  growth.  If  the  conditions  are  favorable  two  or  more  buds 
may  be  seen  arising  from  the  same  individual,  and  occasionally 
a  secondary  bud  may  be  found  arising  from  the  side  of  the  bud, 
even  before  it  has  broken  away  from  the  animal  that  produced 
it.  In  the  case  of  the  Hydra,  however,  these  buds  do  not  remain 
attached  very  long,  but  always  separate;  so  that  we  never  find 
the  animals  grouped  together  in  great  masses.  While  we  may 
find  one  Hydra  with  one,  two,  or  three  buds,  this  is  the  extent 
of  group  formation.  In  closely  allied  animals,  however,  the 
budding  may  go  on  almost  indefinitely,  and  groups  are  formed 


148 


BIOLOGY 


containing  hundreds  of  members,  all  having  arisen  from  the 
original  by  budding.  This  occurs  among  the  hydroids  which 
are  common  at  the  seashore,  examples  of  which  are  shown  in 
Figures  70,  72,  and  73.  In  such  colonies  the  individual  mem- 
bers are  called  zooids. 

Polymorphism. — In  the  colonies  of  hydroids  shown  in  Fig- 
ure 71  the  members  of  the  colony  are  all  alike.  It  not  infre- 
quently happens,  however,  that  when  one  of  these  hydroids 

produces  a  colony  by  budding, 
the  members  (zooids)   assume 


FIG.  70. —  PARYPHA 

An  animal  related  to  Hydra,  but  forming 
colonies  by  budding. 


FIG.  71.  —  ALCYONIUM.  AN  ANI- 

MAL   RELATED     TO      HYDRA 
WHICH    FORMS   COLONIES 


The  individual  members,  which  have 
arisen  by  budding,  are  imbedded  in  a 
lime  base;  p,  one  of  the  members  of  the 
colony  more  highly  magnified. 


forms  unlike  each  other.  In  Figure  72  will  be  seen  a  colony  with 
two  types  of  members;  one  of  them  possessing  tentacles  and 
adapted  for  feeding,  and  the  other  without  tentacles  but  develop- 
ing the  reproductive  bodies  inside  of  a  case.  One  of  these  mem- 
bers is  known  as  the  nutritive  zooid,  nz,  and  the  other  as  the 
generative  zooid,  gz.  In  some  other  types  of  hydroids  the  mem- 
bers which  arise  by  budding  assume  even  a  greater  variety  of 
form.  In  the  colony  shown  in  Figure  73  there  is  a  complicated 


HYDRA  FUSCA 


149 


colony  made  up  of  at  least  five  different  types  of  members  or 
zooids.  Among  them  may  be  found  members  adapted  to  feed- 
ing, n;  others  having  purely  sensory  functions,  called  tentacu- 
lar zooids,  t;  some  adapted  for  reproduction,  g;  others  in  the  form 
of  bells  with  muscles  which  enable  them  to  move  about,  called 
the  swimming  zooids,  sw;  and 
finally  at  the  top  of  the  colony 


e' 


FIG.  72. —  CAMPANULARIA 

A  colony  of  Hydroids  showing  a  differenti- 
ation into  feeding  zooids,  nz,  and  generative 
zooids,  gz;  e,  e,  eggs  in  different  stages  of 
development;  e'  the  young  embryo  extruded 
into  the  water. 


.   73.  -  A    SIPHONOPHORE 


An  animal  showing  a  high  condi- 
tion of  polymorphism;  /,  the  floating 
zooid;  g,  the  generative  zooid  ;  n.the 
nutritive  zooid;  sw,  the  swimming 
zooid;  t,  the  tentacular  zooid. 


a  single  one  develops  as  a  gas  bladder,  /,  which  enables  the 
animal  to  float  in  the  water.  All  of  these  combine  to  form 
a  colony.  Where  several  different  types  are  found  arising  by 
budcfeng  from  the  same  original  stock  the  condition  is  spoken 
of  as  polymorphism  (Gr.  polus  =  many  +  morphe  =  form). 
Polymorphism  is  best  illustrated  in  simple  organisms,  being  well 
developed  among  the  animals  related  to  the  Hydra;  but  the  samp 


150  BIOLOGY 

principle  is  found  in  a  less  developed  extent  in  some  of  the  organ- 
isms with  a  higher  structure,  though  nowhere  do  we  find  it  so 
highly  developed  as  among  the  hydroids.  Where  polymorphism 
is  developed  the  whole  colony  acts  as  a  unit,  and  the  colony, 
therefore,  may  be  compared  to  a  more  highly  complex  organ- 
ism with  its  various  organs.  Polymorphism  always  arises  as 
the  result  of  asexual  growth  and  not  by  sexual  reproduction, 
and  when  it  occurs  the  members  of  the  colony  always  show  a 
differentiation  in  function  as  well  as  in  shape  and  structure. 

Regeneration  of  Lost  Parts. — Hydra  has  a  wonderful  power 
of  reproducing  lost  parts.  If  it  is  cut  into  two  pieces,  each 
part  will  develop  the  part  that  it  has  lost  and  becomes  a 
new  Hydra.  Indeed,  it  may  be  cut  into  a  large  number  of  frag- 
ments, and  every  fragment  is  capable  of  growing  and  developing 
into  a  new  form  like  that  of  which  it  was  originally  a  part. 
If  the  small  conical  projection  containing  the  tentacles  is  cut 
off  from  the  rest  of  the  Hydra,  each  piece  will  develop  the  part 
that  it  has  lost.  The  animal  may  be  split  lengthwise  into  two 
or  four  parts  and  each  will  become  a  perfect  animal.  If  a  head  is 
split  in  two  and  the  parts  slightly  separated,  each  will  develop 
its  crown  of  tentacles  and  a  two-headed  animal  will  result.  If 
an  animal  is  turned  wrong  side  out,  it  will  adjust  itself  to  new 
conditions  and  a  perfect  animal  will  soon  be  produced.  This 
power  of  regenerating  lost  parts  is  found  in  many  of  the  lower 
animals,  but  in  no  place  is  it  better  developed  than  in  Hydra. 
In  the  higher  animal  the  power  of  regenerating  lost  parts  eventu- 
ally disappears  entirely.  It  is  very  evident  that  this  power  must 
be  of  considerable  advantage  to  the  animal  in  the  struggle  for 
existence.  In  Hydra  the  power  is  so  extraordinarily  developed 
that  a  piece  of  the  animal  not  more  than  one-hundredth  of  an 
inch  in  length  is  capable  of  reproducing  all  of  the  parts  that  are 
lacking  and  developing  into  a  new  animal.  In  some  cases  the 
new  animal  is  produced  by  a  multiplication  of  cells  from  these 
pieces,  so  that  a  fair-sized  animal  is  developed;  while  in  other 
cases  the  cells  and  fragments  are  remolded  into  new  icdividuals 


HYDRA  FUSCA  151 

which  are  like  the  original  in  shape  but  much  smaller  in  size. 
Some  of  the  experiments  described  were  originally  performed 
long  ago,  by  Trembley  in  1740 ;  but  they  have  since  been 
confirmed  by  other  investigators. 

Sexual  Reproduction. — By  the  method  of  budding  Hydra 
may  multiply  indefinitely  as  long  as  it  has  plenty  of  food.  It 
has  also  a  second  method  of  reproduction  by  a  true  sexual  proc- 
ess. Under  certain,  not  well-understood,  conditions  the  animal 
produces  outgrowths  on  its  side,  shown  in  Figure  69  C,  which 
are  the  sexual  glands, —  ovaries,  o,  and  spermaries,  s.  Within 
them  are  produced  special  cells,  called  eggs  and  sperms,  which 
unite  with  each  other  in  a  manner  similar  to  that  seen  in  the 
cells  of  Pandorina  (page  74).  The  significance  of  this  repro- 
duction will  be  noticed  in  a  later  chapter. 

Hydra,  as  will  be  seen  from  the  above  description,  possesses 
the  systems  of  alimentation,  metabolism,  motion,  and  reproduc- 
tion. Circulation  is  wanting;  respiration  is  carried  on  through 
the  general  surface  of  the  cells;  no  excretory  system  is  found,  each 
cell  probably  excreting  its  waste  products  directly  into  the 
water;  support  is  unnecessary  in  such  a  small  animal;  the  rudi- 
ments of  nerves  suggest  the  beginning  of  a  coordinating  system. 

THE  RELATION   OF  THE  WHOLE   ORGANISM   TO   ITS  DIFFER- 
ENT PARTS 

With  the  appearance  of  multicellular  organisms  we  also  find 
that  the  entire  animal  has -now  a  life  more  or  less  independent 
of  the  life  of  its  parts.  The  multicellular  animal  or  plant  lives 
a  life  as  a  complex,  and  in  addition  each  cell  has  a  life  of  its  own; 
so  that  we  can  distinguish,  in  a  multicellular  animal,  a  life  of 
the  organism  as  a  whole  and  a  life  of  its  separate  cells.  It  is 
possible  for  the  death  of  the  organism  as  a  complex  to  occur 
while  the  individual  cells  still  remain  alive.  It  is  true  that  in  the 
multicellular  organism  each  of  the  individual  cells  is  dependent 
upon  the  activity  of  the  whole  to  keep  it  properly  nourished  and 
supplied  with  the  necessary  conditions  of  its  life.  The  different 


152  BIOLOGY 

cells  that  make  up  such  organisms  are  not  independent  and  can- 
not live  long  except  when  related  to  the  other  cells  that  make  up 
the  multicellular  organism.  Nevertheless,  there  is  a  certain 
amount  of  independence  in  the  individual  cells,  especially  among 
plants  and  some  of  the  lowest  animals;  for  in  these  we  may 
remove  only  a  comparatively  small  number  of  cells  from  the 
whole  organism  and  these  cells  will  still  retain  their  vitality,  still 
continue  their  power  of  growth,  and  under  proper  circumstances 
develop  more  cells  which  eventually  become  exactly  like  the 
animal  from  which  they  were  obtained.  This  is  especially  true 
of  Hydra,  which  can  be  cut  into  many  pieces,  each  piece  retain- 
ing the  power  of  independent  life,  and  in  time  becoming  an  inde- 
pendent and  well-developed  animal.  In  such  low  organisms  the 
life  of  the  organism  as  a  complex  has  not  wholly  destroyed  the 
independence  of  the  individual  parts.  This  is  more  or  less  true 
throughout  the  whole  of  the  plant  kingdom.  Among  most  higher 
plants  as  well  as  the  lower,  small  pieces  separated  from  the 
parent  plant  will  not  die  at  once,  but  may,  if  put  under  proper 
conditions,  develop  into  fully  grown  individuals  like  those  from 
which  the  fragments  were  obtained. 

With  animals,  however,  it  is  only  among  the  lowest  and 
simplest  forms  that  a  piece,  containing  a  relatively  small 
number  of  cells,  can  be  separated  from  the  rest  and  still  be 
capable  of  developing  into  a  new  organism  like  the  original,  as 
in  the  case  of  the  Hydra.  As  we  pass  to  the  higher  animals 
this  power  of  regeneration  disappears,  and  among  almost  all 
animals,  even  of  comparatively  low  structure,  the  independent 
life  of  the  parts  is  lost,  so  that  when  one  portion  is  removed 
from  the  complex  that  makes  up  the  animal  it  no  longer  retains 
its  power  of  life  and  growth.  But  even  in  these  cases  and  among 
the  highest  animals,  we  do  find  that  some  parts  may  have  more 
or  less  independent  life  when  separated  from  the  organism  of 
which  they  are  a  part.  In  an  animal  like  a  frog,  for  example, 
the  heart  may  be  totally  removed  from  the  body  and  it  will  still 
keep  up  its  life  for  many  hours  when  put  under  proper  condi- 


HYDRA  FUSCA  153 

tions,  long  after  the  frog  itself  has  been  killed.  More  remarkable 
is  this  power  in  the  case  of  a  turtle,  for  here  even  when  the  animal 
has  its  head  entirely  cut  from  the  body,  and  the  rest  of 
the  animal  destroyed,  the  heart,  if  removed  and  kept  under 
proper  conditions,  will  keep  on  beating  for  at  least  two  days. 
Still  more  remarkable  is  it  to  find  that  in  the  air  passages  of  the 
turtle  there  are  ciliated  cells  which  have  a  special  power  of  motion ; 
Fig.  14  C.  During  all  the  life  of  the  turtle  these  cilia  are  in  a 
state  of  active  motion,  and  after  the  turtle  is  dead  the  cilia 
may  continue  moving  for  as  long  as  two  weeks.  We  thus  see 
that  among  the  higher  organisms  the  death  of  the  animal  as  a 
whole  does  not  necessarily  involve  an  immediate  death  of  all  its 
parts.  The  individual  parts  are,  of  course,  closely  dependent 
upon  each  other,  and,  at  least  in  the  higher  organisms,  the  life 
of  neither  is  capable  of  being  long  maintained  without  the  other; 
but  the  life  of  the  individual  cell  may  frequently  continue  some 
time  after  the  life  of  the  organism  as  a  whole  has  been  brought 
to  an  end. 

From  this  it  follows  that  the  term  death  may  have  a  different 
meaning  in  different  connections.  In  speaking  of  the  death  of  an 
animal,  we  may  refer,  and  usually  do  refer,  to  the  death  of  the 
animal  as  a  whole,  which  means  the  destruction  of  the  compli- 
cated mechanism  that  forms  the  animal  organism.  But  we  may 
also  refer  to  the  death  of  the  individual  parts,  and  in  this  case 
the  exact  time  when  the  animal  comes  to  its  death  is  difficult 
to  state.  The  animal  as  a  whole  may  die  on  one  day,  while 
some  of  its  parts  may  remain  alive  at  least  two  weeks.  In 
such  instances  it  is  not  easy  to  say  when  death  occurs.  Never- 
theless, it  is  customary  to  refer  by  this  term,  not  to  the  death 
of  the  individual  parts  or  the  individual  cells  which  make 
up  the  animal,  but  to  the  destruction  of  the  organism  as  a 
whole,  which  causes  it  to  cease  to  act  as  a  unit.  Usually, 
therefore,  death  refers  to  the  breaking  down  of  the  mecha- 
nism of  which  an  organism  is  composed  so  that  its  parts  do 
not  act  together. 


154  BIOLOGY 


LABORATORY  WORK 

Hydra. — Almost  any  pond  will  furnish  Hydra,  which  may  be  found 
clinging  to  the  under  side  of  pond-lily  leaves.  If  such  leaves  are  placed  in 
a  dish  of  clean  water,  the  Hydra  will  detach  themselves  from  the  leaves  and 
cling  to  the  side  of  the  dish.  For  study,  a  specimen  is  to  be  detached  from 
the  dish,  placed  in  a  watch  glass  containing  a  little  water,  and  examined 
under  the  microscope  with  a  low-power  objective.  The  general  structure 
and  motion  of  the  animals  may  easily  be  seen.  For  the  cellular  structure 
of  the  body,  stained,  mounted  sections  should  be  furnished  the  student  by 
the  instructor.  For  a  study  of  the  nematocysts,  a  bit  of  the  tentacles  of  a 
brown  Hydra  should  be  cut  off  with  delicate  scissors  and  placed  on  a  slide 
in  a  small  drop  of  water.  A  cover  glass  is  placed  upon  the  drop  and  gently 
pressed.  This  will  crush  the  tentacle  and  cause  many  of  the  nematocysts 
to  discharge  their  stinging  hairs.  The  nematocysts  may  also  be  made  to 
discharge  their  stinging  hairs  if  a  little  weak  acetic  acid  is  added  .to  the 
water.  A  careful  examination  with  a  |-inch  objective  will  show  all  three 
kinds  of  nematocysts,  both  discharged  and  undischarged.  For  comparison 
of  Hydra  with  other  Hydroids,  preserved  and  mounted  specimens  should 
be  furnished  by  the  instructor,  some  of  which  should  show  colonies  and 
others  jellyfishes. 

BOOKS  FOR  REFERENCE 

COLTON,  Zoology,  Descriptive  and  Practical,  D.  C.  Heath  &  Co.r 
Boston. 

PRATT,  Invertebrate  Zoology,  Ginn  &  Co.,  Boston. 

MARSHALL  and  HURST,  Practical  Zoology,  G.  P.  Putnam's  Sons,  New 
York. 


CHAPTER  VIII 

MULTICELLULAR  ANIMALS:  THE  EARTHWORM  (LUM- 

BRICUS) 

THE  earthworm  is  an  extremely  common  animal  the  world 
over,  being  found  buried  in  moist  earth  in  practically  all  parts 
of  the  world.  There  are  numerous  species,  differing  from  each 
other  in  minor  details,  but  agreeing  in  their  fundamental 
structure.  The  animals  vary  in  size  from  those  an  inch  or  two 
in  length,  to  some  which  are  nearly  a  foot;  and  one  species 
is  reported  two  feet  in  length.  Earthworms  are  of  practical 
importance  in  stirring  up  the  soil.  They  are  constantly  en- 
gaged in  bringing  soil  from  below  to  the  surface,  and  depositing 
it  at  the  mouths  of  their  burrows.  By  this  slow  but  constant 
action  they  are  of  much  value  to  agriculture,  constantly  renew- 
ing the  surface  soil. 

ANATOMY 

Shape  of  Body. — Examined  externally,  the  earthworm  is  an 
elongated  animal,  more  or  less  cylindrical  in  shape,  tapering, 
however,  at  the  two  ends;  Fig.  74.  The  head,  or  anterior 
end,  is  more  tapering  than  the  other,  the  blunter  one  being 
the  posterior  end.  One  side  of  the  animal  is  lighter  colored 
than  the  other  and  slightly  flattened,  the  opposite  side  being 
more  rounded.  When  the  animal  is  in  its  natural  position 
on  the  surface  of  the  ground,  the  flat  side  is  kept  undermost 
and  the  rounded  and  darker-colored  side  uppermost.  We  thus 
have  an  anterior  and  a  posterior  end,  a  ventral  and  a  dorsal 
surface,  and,  consequently,  a  right  and  left  side  to  the  animals. 
The  animal  is,  therefore,  bilaterally  symmetrical. 

Segments  or  Metameres. — The  body  of  the  earthworm  is 
divided  into  a  number  of  rings  (Fig.  74)  called  segments  or 
metameres  (Gr.  meta  =•  after  -f  meros  =  part).  The  number 
is  not  constant,  being  greater  in  the  older  and  larger  animals 

155 


156 


BIOLOGY 


than  in  the  younger  ones,  and  increasing  with  age.  Most 
of  these  rings  are  alike  in  shape  and  size, 
but  a  few  of  them  differ  slightly  from  the 
others.  The  first  one  at  the  anterior  end 
is  not  a  complete  ring,  but  a  minute  pro- 
jection which  is  known  as  the  prostomium 
(Gr.  pro  =  before  +  stoma  =  mouth).  It  is 
slightly  movable  and  is  the  most  sensitive 
part  of  the  animal.  The  second  segment  is 
not  a  complete  ring,  but  rather  in  the  form 
of  a  horseshoe,  with  the  open  part  of  the 
horseshoe  above, 
and  with  the 
prostomium  lobe 
fitting  into  the 
opening  as  shown 
in  Figure  75. 

Underneath 
the  prostomium 
and  over  the  sec- 
ond segment  is 
an  opening,  the 
mouth,  m.  The 
third  segment  is 
a  complete  ring, 
but  rather  small, 
and  from  this 
point  backwards 
the  segments  are 

all  alike  in  shape,  increasing  slightly  in  size 
until  a  maximum  is  reached,  and  from  this 
point  remaining  essentially  the  same  in  size 
and  shape  to  the  posterior  end  of  the  body. 
A  short  distance  back  from  the  head  there 
is  a  series  of  rings,  from  the  twenty-eighth 


FIG.  74.  — AN 
EARTHWORM, 
FROM  BELOW  AND 
FROM  THE  SIDE 

a,  the  anus;  m , 
mouth;  od,  opening  of 
oviduct;  sr,  opening  of 
seminal  receptacles;  v, 
opening  of  vasdeferena; 
a,  aetse. 


FIG.  75. —  THE  FIRST  THREE 
SEGMENTS  OF  THE  EARTH- 
WORM 

A,  from  the  front;  B,  from  the 
side;  p,  the  prostomial  lobe;  m,  the 
mouth. 


THE  EARTHWORM 


to  the  thirty-fifth  segments,  known  as 
the  clitellum  (Lat.  clitellce  =  saddle) ; 
Fig.  74.  These  segments  are  larger  than 
elsewhere  and  have  a  thicker  wall  and 
special  functions.  At  the  extreme  pos- 
terior end  the  segments  become  smaller, 
and  the  last  one  has  an  opening  which 
is  the  posterior  opening  of  the  diges- 
tive tract,  the  vent  or  anus,  a.  Be- 
cause of  this  ringed  structure  the 
earthworm  belongs  to  a  class  of  animals 
called  Annulata  (Gr.  annulus  =  ring). 
Structure  of  the  Body. — The  body 
of  the  earthworm  can  be  compared  to 
a  tube  within  a  tube;  Fig.  76.  The 
outer  tube  is  called  the  body  wall,  6, 
and  the  inner  tube  the  alimentary 
canal  or  the  digestive  tract.  Between 
the  body  wall  and  the  digestive  sys- 
tem is  a  space  filled  with  a  liquid,  this 
space  being  a  true  body  cavity  or  coelom 
(Gr.  koilos  =  hollow),  c,  differing  thus 
from  Hydra,  that  has  no  coelom.  The 
body  cavity  is  not,  however,  an  open 
space  extending  from  the  anterior  to 
the  posterior  end,  but  is  divided  by 
partitions  into  a  series  of  chambers, 
with  a  chamber  for  each  segment.  The 
partitions  are  called  septa  (sometimes 
called  dissepiments) .  There  are  minute 
openings  through  each  septum,  so  that 
the  liquid  that  fills  the  body  cavity 
may  pass  through;  thus  the  different 
chambers  are  in  communication  with 
each  other. 


FIG.  76. —  DIAGRAM  SHOW- 
ING THE  ANTERIOR  END 
OF  THE  EARTHWORM  CUT 
LENGTHWISE  THROUGH  A 
VERTICAL  MEDIAN  LINE 

6,  body  wall;     od,  oviduct; 


c,  coalom; 
cr,  crop; 
g,  gizzard; 
m,  mouth; 
mu,  muscles; 
n,  nephridia; 
o,  ovary; 


of,  (esophagus; 
ph,  pharynx; 
s,  septa; 
sp,  spermary ; 
sr,  seminal  recep- 
tacles ; 
t,  typhlosole. 


158  BIOLOGY 

The  Alimentary  Canal. — The  alimentary  canal  (enteron)  is 
a  straight  tube  extending  from  one  end  of  the  animal  to  the 
other,  without  any  convolutions.  It  does,  however,  show 
several  distinct  regions.  The  mouth  opens  into  a  slightly 
swollen  section  known  as  the  throat  or  pharynx,  ph.  The 
pharyngeal  walls  are  muscular,  with  a  radiating  series  of 
muscles  that  pass  outward  to  be  attached  to  the  body  wall, 
mu.  The  contraction  of  these  muscles  will  cause  an  expansion 
of  the  pharynx  and  convert  it  into  a  sucking  organ  by  means 
of  which  the  animal  draws  food  into  its  mouth.  Behind  the 
pharynx  the  canal  contracts  into  a  straight  gullet  or  oesopha- 
gus, oe,  which  continues  back  to  the  fifteenth  segment.  Here 
it  enlarges  into  a  thin-walled  crop,  cr,  which  is  followed  in 
the  fifteenth  and  seventeenth  segments  by  a  second  enlarge- 
ment with  thicker  walls,  called  the  gizzard,  g.  Beyond  this 
the  intestine  extends  in  a  straight  line  to  the  anal  aperture 
or  vent.  The  intestine  is  not  a  simple  cylindrical  tube  but  has 
its  dorsal  side  folded  inward  to  form  a  longitudinal  ridge  known 
as  the  typhlosole  (Gr.  typhlos  =  blind  +  solen  =  tube),  ty  (Fig. 
81),  whose  purpose  seems  to  be  only  to  increase  the  amount 
of  interior  surface  within  the  intestine. 

Circulatory  System. — The  circulatory  system  consists  of  two 
parts,  the  blood  system  and  the  codomic  fluid. 

The  blood  system. — A  series  of  tubes  or  vessels  containing 
blood  comprises  the  circulatory  system.  The  blood  of  the 
earthworm  is  red,  a  very  unusual  condition  among  lower  ani- 
mals. The  red  color  is  due  to  a  substance  called  haemoglobin 
(Gr.  haima  =  blood  +  Lat.  globus  =  globe),  which  is  dissolved 
in  the  liquid  part  of  the  blood,  and  is  not  contained  in  the 
corpuscles,  as  it  is  in  the  frog  and  higher  animals.  This  blood 
is  kept  in  constant  motion  in  the  vessels,  forced  along  by  their 
contractions.  The  chief  vessels  and  the  direction  of  the  blood 
current  are  shown  in  Figure  77  and  they  are  as  follows:  — 

Running  anteroposteriorly,  just  above  the  alimentary  tract, 
is  a  large  longitudinal  dorsal  vessel,  dv,  with  muscular  walls. 


THE  EARTHWORM 


159 


These  muscles  produce  waves  of  contraction,  which,  arising  at 
the  posterior  end,  force  the  blood  forward.  In  the  posterior 
half  of  the  body  small  branches  pass  from  this  tube  into  the 
intestine,  ei,  supplying  its  walls,  and  the  blood  then  enters 
a  rather  large  vessel  in  the  typhlosole,  from  which  it  passes 
back  by  short  tubes,  ai,  into  the  dorsal  vessel.  The  greater 
part  of  the  blood  in  the  dorsal  vessel  flows  forward  to  the  seg- 
ments 6-11,  where  five  large  circular  vessels  arise  from  it, 
ht,  which  pass  around  the  sides  of  the  body  to  enter  a  sub- 
intestinal  vessel,  w,  also  extending  lengthwise  and  lying  be- 

Uv 


FIG.  77. —  DIAGRAM  SHOWING  THE  CHIEF  BLOOD 
VESSELS  OF  THE  EARTHWORM 


an,  the  anterior  end; 

po,  the  posterior  end  of  the  body; 

dv,  dorsal  vessels; 

cv,  circular  vessels; 

ei,  efferent  intestinal; 

(Bourne  and  Benham.) 


ai,  afferent  intestinal; 
ht,  hearts; 

snv,  subneural  vessel; 
vv,  ventral  vessel. 


neath  the  intestine.  These  circular  vessels  are  called  hearts, 
since  they  contract,  and  force  the  blood  downward  into  the 
ventral  vessel.  When  reaching  the  ventral  vessel,  part  of  the 
blood  flows  forward,  in  front  of  the  hearts,  and  part  of  it  back- 
ward. From  this  ventral  vessel  branches  arise  which  pass  out 
into  the  body  wall  and  into  other  organs  supplying  the  body 
generally  with  blood.  After  passing  through  the  organs  of 
the  body  wall,  etc.,  the  blood  is  collected  into  another  set  of 
vessels  which  pass  into  a  third  longitudinal  vessel  lying  under 
the  nerve  chord,  the  subneural  (Gr.  neuron  =  nerve),  snv 
Through  this  it  flows  toward  the  posterior  end.  In  the  intestinal 
region  there  arises  from  the  subneural  vessel,  in  each  segment, 
a  circular  vessel,  cv,  which  passes  up  around  the  body  to  empty 


160  BIOLOGY 

into  the  dorsal  vessel,  dv,  thus  bringing  the  blood  back  again 
into  the  dorsal  vessel.  There  are  numerous  other  small  ves- 
sels, some  of  which  are  shown  in  Figure  77,  but  the  chief  ones 
are  those  that  have  been  described. 

The  blood  is  forced  onward  by  the  contraction  of  the  walls 
of  the  dorsal  vessel  and  the  hearts,  which  are  provided  with 
valves  preventing  any  back  flow  when  the  contractions  occur. 
The  course  of  the  blood  is  rather  indefinite  and  the  pure  and 
impure  blood  are  not  distinctly  separated  from  each  other,  as 
in  higher  animals.  There  are  no  true  arteries  or  veins,  and  no 
true  hearts.  This  blood  is  associated  With  respiration,  and  also 
carries  nourishment  from  the  absorbing  organs  in  the  intestine 
to  the  active  tissues,  and  carries  waste  products  from  the  active 
cells  to  the  excreting  organs. 

Coelomic  or  Perivisceral  Fluid. — The  chambers  of  the  body 
cavity  are  filled  with  a  fluid  called  the  ccelomic  or  perivisceral 
(Gr.  peri  =  around  -fLat.  viscera  =  internal  organs)  fluid,  which 
serves  also  as  a  circulatory  medium.  The  food  that  is  absorbed 
makes  its  way  into  the  body  cavity  and  is  partly  absorbed 
by  this  fluid.  This  liquid  is  forced  irregularly  backward  and 
forward  through  the  cavity  of  the  body  by  the  motions  of 
the  animal,  and  the  nutritious  parts  of  the  food  which  are  dis- 
solved in  it  are  thus  directly  carried  to  and  fro  and  brought 
in  contact  with  the  living  tissues  of  the  body,  that  are  bathed 
in  this  liquid.  There  is  no  distinct  circulation  of  this  fluid,  and 
it  cannot  properlybe  called  a  circulatoryfluid.  It  does,  however, 
have  some  of  the  functions  of  the  blood,  since  it  carries  to  and 
fro  a  part  of  the  material  absorbed  from  the  digestive  tract. 
It  corresponds  more  closely  to  the  lymph  of  higher  animals. 

Respiration. — The  earthworm  has  no  distinct  respiratory 
system,  but  the  blood  vessels  in  their  circulation  in  the  skin 
are  brought  into  a  very  close  proximity  with  the  air.  Gases  are 
readily  exchanged  through  the  thin  skin,  and  respiration  is 
carried  on  easily  without  any  special  respiratory  organs  except 
the  minute  blood  vessels  that  lie  beneath  the  skin. 


THE  EARTHWORM 


161 


Excretory  System. — Most  of  the  excreted  matter  (with  the 
exception  of  gases)  is  passed  to  the  exterior  by  a  series  of  tubes 
known  as  nephridia  (Gr.  nephros  =  kidney),  one  pair  in  each 
segment.  Each  of  them  (see  Fig.  78)  consists  of  a  long  tube, 
which  begins  in  a  segment  of  the  body  cavity  as  a  minute 
funnel-shaped  opening,  i,  and  then  passes  through  the  septa,  s, 
to  the  segment  immediately  behind.  In  the  posterior  segment, 
the  tube  is  coiled  back  and  forth  in  three  distinct  loops  that 
differ  in  structure  and  function.  Eventually  the  distal  end 
passes  through  the  walls  of  the  body  to  the  exterior,  by  a 
lateral  opening,  e,  in  each 
segment.  These  nephridia 
are  very  delicate  organs  and 
can  only  be  made  out  by 
very  careful  study  with  a 
magnifying  glass.  Their 
function  in  excretion  is  as 
follows:  The  funnel  opening 

in  the    anterior    segment    is  „      __ 

....  ...  FIG.  78. —  A  NEPHRIDIUM,  COMPLETE 

guarded  with  cells  provided 

with  cilia,  and  some  of  the 
coils  are  also  lined  with  cilia. 
The  movements  of  these  cilia  produce  currents  in  the  liquid 
in  the  tube  and  force  the  liquids  through  the  tube  to  the 
exterior.  As  a  further  result  of  the  action  of  these  cilia,  solid 
particles  of  waste  material,  which  may  be  floating  in  the 
ccelomic  fluid,  are  forced  into  the  tube  and  then  through 
the  tube,  passing  through  its  coils  and  finally  reaching  the 
exterior  through  its  opening.  The  coiled  walls  of  the  tube 
are  made  up  of  thick  active  cells  which  are  well  supplied  with 
blood  vessels.  These  are  secreting  cells  and  resemble  gland 
cells.  They  have  the  power  of  extracting  waste  products  from 
the  blood  and  excreting  them  into  the  tube  which  they  surround. 
The  materials  enter  the  duct  of  this  nephridium  and  are  slowly 
forced  along  by  the  ciliary  current,  and  finally  carried  to  the 


i,  incurrent  opening; 
e,  excurrent  opening; 
s,  septa. 


162 


BIOLOGY 


exterior.  These  nephridia  have  as  their  primary  function  the 
removing  from  the  body  of  the  waste  products  containing 
nitrogen,  related  to  urea.  Their  function  is  thus  similar  to 
that  of  the  kidneys  of  the  higher  animals,  and  indeed  their 
structure  is  not  unlike  the  kidneys  of  some  of  the  ver- 
tebrates. 

The  Coordinating  or  Nervous  System. — The  nervous  system 
consists  of  a  central  system  and  a  peripheral  (Gr.  peri  =  around 


Fia.  79. —  DIAGRAM  SHOWING  THE  NERVOUS  SYSTEM 

IN  THE  FRONT  END  OF  THE  BODY 


eg,  cerebral  ganglia; 
com,  commissures; 
m,  mouth; 


oe,  oesophagus; 
pr,  prostomium; 
v,  ventral  cord. 


(Shipley  and  MacBride.) 

+  pherein  =  to  bear)  system,  the  latter  composed  of  a  large 
number  of  nerves  passing  from  the  central  system  into  the 
various  regions  of  the  body. 

The  central  system. — 1.  The  cerebral  ganglia.  These  are 
two  nerve  knots  or  ganglia,  sometimes  called  the  brain,  united 
together  and  lying  above  the  pharynx  in  the  anterior  part  of 
the  body  cavity;  Fig.  79  eg.  From  them,  extending  down- 
ward and  backward,  a  pair  of  cords  or  commissures  (Lat. 
committere  —  to  join  together),  com,  pass  around  the  phar- 
ynx and  unite  with  each  other  below  on  the  ventral  side 


THE  EARTHWORM  163 

of  the  pharynx  a  short  distance  behind  the  mouth.  2.  The 
ventral  cord.  When  the  two  commissures  have  united  they 
form  a  cord  which  passes  to  the  posterior  end  in  the  median 
line  of  the  body,  closely  attached  to  the  body  wall  beneath 
the  intestines;  this  is  the  ventral  cord,  v.  In  each  segment 
the  cord  is  slightly  enlarged  to  form  what  is  called  a  ganglion; 
see  Fig.  80  vc.  At  the  posterior  end  of  the  body  this  cord 
becomes  smaller  and  finally  terminates. 

The  peripheral  system. — The  nerves  which  form  the  per- 
ipheral system  are  numerous.  From  the  cerebral  ganglion 
two  large  nerves  arise,  which  soon  divide  into  many  branches 
and  pass  forward  to  the  prostomium,  giving  it  a  very  large 
nerve  supply  and  making  it  a  very  sensitive  organ;  Fig.  79. 
From  the  commissures  extending  around  the  oesophagus  arise 
the  nerves  that  supply  the  second  and  third  segments  of  the 
body.  From  the  ventral  cord  in  each  of  the  segments,  from 
the  fourth  to  the  posterior  end  of  the  body,  there  arise  three 
pairs  of  nerves.  Two  pairs  arise  from  the  ganglionic  enlarge- 
ment and  one  pair  from  the  sides  of  the  ventral  cord  behind 
the  septum  that  separates  each  segment  from  the  next. 

Reproductive  System. — The  only  method  of  reproduction  in 
the  earthworm  is  by  sexual  process.*  The  two  sexes  are,  how- 
ever, combined  in  the  same  individual,  so  that  the  earthworm 
is  what  is  called  an  hermaphrodite;  see  page  251. 

Female  reproductive  organs. — In  the  thirteenth  segment 
there  is  a  pair  of  small  glands  called  ovaries,  situated  on  the 
ventral  side  of  the  body  cavity  close  to  the  middle  line;  Fig. 
80  ov.  In  the  same  segment  is  the  opening  of  a  funnel  which 
leads  into  a  short  tube  passing  through  the  septa  into  the  next 
posterior  segment.  Here  it  is  slightly  enlarged  to  form  an  egg 
sac,  and  from  the  sac  a  small  duct  extends  through  the  body 
wall  to  the  exterior,  opening  upon  the  ventral  surface  of  the 
fourteenth  segment.  These  ducts  are  the  oviducts,  od,  and 
through  them  the  eggs  produced  by  the  ovary  pass  to  the 

*The  earthworm  has  a  slight  power  of  regeneration  of  lost  parts,  but  this  power  is  far  less 
developed  than  in  Hydrg,,     If  it  is  cut  into  two  pieces  two  individuals  are  formed. 


164 


BIOLOGY 


exterior.     The  openings  of  the  reproductive  organs  may  be 
seen  in  Figure  74. 

Male  reproductive  organs. — In  the  tenth  and  eleventh  seg- 
ments there  is  a  pair  of  glands,  the  spermaries,  sp,  in  which 
are  formed  the  male  reproductive  elements.  In  these  two  seg- 
ments their  position  corresponds  to  the  position  of  the  ovary 
in  the  thirteenth  segment.  They  are  very  small  glands  and 
can  only  be  seen  by  microscopic  examination.  Behind  each 

of  these  sperm  glands 
is  a  funnel-shaped,  cili- 
ated opening,  leading 
into  a  tube  which 
passes  through  the 
septa  into  the  next  seg- 
ment, where  it  is  slightly 
coiled,  and  then  passes 
backward.  The  tubes 
from  the  two  sperm 
glands  on  each  side 
unite  with  each  other 
in  the  twelfth  segment 
to  form  a  single  duct, 
which  passes  back 
through  the  septa  to  the 
fifteenth  segment,  where 
it  opens  through  the 
body  wall  to  the  exte- 
rior. This  duct  is  known 
as  the  vas  deferens 
(Lat.  vasa  =  vessel  -f- 
deferens  =  carrying 


FIG.  80. —  DIAGRAM  SHOWING  THE  REPRO- 
DUCTIVE SYSTEM  OF  THE  EARTHWORM 
The  numbers  represent  the  number  of  segments, 
seminal  receptacle 
seminal  vesicles; 


es,  egg  sac;  sr,  seminal  receptacles; 

ne,  nephridia;  '     ' 


OB,  ovary; 
od,  oviduct; 
sp,  spermaries; 


vc,  ventral  nerve  cord; 
vd,  vasdeferens. 


down);  Fig.  80  vd.  In  the  ninth,  tenth,  and  eleventh  seg- 
ments are  large  sacs  known  as  seminal  vesicles,  sv,  which  serve 
as  a  storehouse  for  the  secretion  of  the  sperm  glands,  before 
these  secretions  pass  to  the  exterior  through  the  vas  def- 


THE  EARTHWORM  165 

erens.  At  the  junction  between  the  ninth  and  tenth,  and 
between  the  tenth  and  eleventh  segments,  may  be  found  two 
pairs  of  white  sacs,  each  opening  to  the  exterior  by  an  opening 
at  the  junction  line  between  the  segments.  These  are  the 
seminal  receptacles,  sr,  and  their  function  is  to  receive  the 
secretions  from  the  seminal  glands  in  copulation. 

Copulation  and  Egg  Laying. — Although  the  earthworm  is  an 
animal  producing  both  male  and  female  elements  in  the  same 
individual,  the  habits  of  the  animal  are  such  that  there  is  no 
fertilization  of  the  egg  by  the  sperm  of  the  same  individual 
that  produces  the  egg,  but  a  cross  fertilization  always  occurs 
between  two  individuals.  At  the  breeding  season,  which  is 
early  in  the  summer,  two  individuals  place  themselves  side  by 
side  with  their  heads  in  opposite  directions,  and  by  means  of  the 
secretions  from  the  glands  in  their  skin  there  is  formed  a  slimy 
covering  that  holds  the  two  individuals  in  close  contact  (copu- 
lation). In  this  position,  each  transfers  sperm  material  (see 
Chapter  XII)  from  its  sperm  glands  into  the  seminal  receptacles 
of  the  other,  after  which  they  separate.  During  copulation, 
or  immediately  afterwards,  a  secretion  is  produced  by  the  cli- 
tellum,  which  forms  a  band  around  the  animal  that  extends 
from  the  twenty-eighth  to  the  thirty-fifth  segment  of  the  body. 
At  the  close  of  copulation,  after  the  animals  have  separated, 
this  band  is  gradually  pushed  forward  until  it  finally  slips  off 
over  the  head.  As  the  band  passes  forward  over  the  fourteenth 
segment  a  certain  number  of  eggs  are  extruded  into  it  from  the 
oviduct;  and  when  it  passes  over  the  ninth  and  tenth  segments 
some  of  the  sperm  material  from  the  seminal  receptacles  is 
also  ejected  into  it.  As  it  passes  off  over  the  head  it  closes 
up  by  its  own  elasticity.  Inside  of  this  band  the  eggs  of  each 
individual  are  thus  mixed  with  the  sperm  from  the  other  indi- 
vidual and  cross  fertilization  occurs.  This  case  holding  the  eggs 
and  sperms  is  now  known  as  a  cocoon,  and  within  it  the 
eggs  develop  into  earthworms.  The  cocoons  are  deposited  in 
the  soil  and  may  be  found  early  in  the  summer. 


.00 


BIOLOGY 


MICROSCOPIC  ANATOMY  OR  HISTOLOGY 

The  body  of  the  earthworm  is  made  of  large  numbers  of 
cells  of  great  variety  in  form  and  structure.  The  cellular 
structure  in  some  of  the  organs  of  the  body  can  readily  be 
made  out  under  the  microscope,  but  in  others  the  cells  can 
be  seen  only  by  special  methods.  The  most  important  features 
of  the  histology  are  as  follows:  — 

Body  WalL — The  body  wall  contains  several  layers;  Fig. 
81.  On  the  outside  a  very  thin  cuticle  covers  the  whole  body, 


3 


FIG.  81. —  DIAGRAM  REPRESENTING  A 

OF  THE  EARTHWORM'S  BODY 


of  the  nephridium; 


end; 


perforated,  however,  by  numerous  openings  through  which  the 
various  secretions  pass.  Inside  of  the  cuticle  is  a  somewhat 
thicker  layer  of  cells  mainly  cylindrical  in  form,  known  as 


THE  EARTHWORM 


167 


the  epidermis,  ep.  Some  of  the  cells  are  sensory  cells;  others 
have  the  power  of  secreting  a  slimy  material  which  keeps  the 
surface  of  the  animal  moist,  and  these  are  called  gland  cells; 
Fig.  82.  Under  the  epidermis  is  a  layer  of  circular  muscles, 
cm,  extending  around  the  body,  each  muscle  in  the  form  of 
a  very  long,  slender  fiber,  tapering  at  both  ends.  Extending 
around  the  bodj*  as  they  do  in  a 
circular  direction,  their  contraction 
will  tend  to  constrict  the  body  and 
reduce  its  diameter.  Under  this  is 
a  thicker  layer  of  muscles,  running 
lengthwise,  the  longitudinal  mus- 
cles, Im.  These  are  arranged  in 
bundles  and  in  a  cross  section  they 
appear  to  radiate  like  a  feather,  but 
each  longitudinal  muscle  fiber  has 
the  same  structure  as  the  circular 
muscles.  By  their  contraction  the 
animal's  body  is  shortened.  Under 
the  longitudinal  muscles  is  an  ex- 
tremely delicate  layer  of  flat  cells 
forming  a  thin  membrane  bound- 
ing  the  body  wall  on  the  surface  lying  next  to  the  body 
cavity.  This  is  the  peritoneal  (Gr.  peri  =  around  +  letnetn  = 
to  stretch)  epithelium,  per. 

Eight  delicate  bristles,  called  setae,  extend  through  the  mus- 
cle layers  of  the  body  wall  and  protrude  through  the  skin, 
Fig.  81  s.  They  are  arranged  in  four  groups,  two  in  each 
segment,  and  are  attached  by  several  minute  muscles  on  the 
inner  end.  By  means  of  these  the  setae  may  be  slightly  ex- 
truded, or  moved  to  and  fro  so  that  the  tips  may  be  directed 
forward  and  backward.  If  the  earthworm  is  pulled  gently 
through  the  fingers,  the  projecting  setae  may  be  felt  as  a  slight 
roughness  on  the  skin. 

Motion. — The  motor  system  of  the  earthworm  is  extremely 


Fie.  82. — HIGHLY  MAGNIFIED 

SECTION    OF     THE    SOX    OF 
THE  EARTHWORM 


168  BIOLOGY 

simple  and  crude,  consisting  only  of  the  two  layers  of  muscles, 
longitudinal  and  circular,  and  the  seta.  The  method  of  its 
action  is  as  follows:  By  the  contraction  of  the  circular  muscles 
the  diameter  of  the  body  is  reduced,  and,  inasmuch  as  the  body 
cavity  is  filled  with  the  perivisceral  liquid,  and  liquids  are 
incompressible,  the  contraction  of  the  diameter  of  the  body 
must  necessarily  increase  its  length.  The  ends  are  thus  pushed 
apart;  but  the  setae  pointed  backward  act  as  anchors,  and 
the  pushing  of  the  two  ends  of  the  body  apart  will  tend  to  push 
the  head  forward,  the  rest  of  the  body  remaining  practically 
stationary.  After  the  contraction  of  the  circular  muscles  the 
longitudinal  muscles  are  contracted,  thus  shortening  the  length 
of  the  body  and  at  the  same  time  increasing  its  diameter. 
As  the  body  shortens,  the  tail  is  pulled  forward  toward  the 
head,  the  setae  again  serving  as  anchors  to  prevent  the  body 
from  moving  in  the  wrong  direction.  Thus  by  alternately  con- 
tracting the  circular  and  longitudinal  muscles,  the  head  is 
pushed  forward  and  the  tail  is  pulled  up  to  the  head.  If  the 
earthworm  wishes  to  move  backward,  it  needs  only  to  contract 
the  muscles  connected  with  the  setae  and  to  point  them  for- 
ward, when  they  will  serve  as  anchors  to  prevent  the  body  from 
being  pushed  forward;  and  the  alternate  contraction  of  the 
two  layers  of  muscles  will  make  the  animal  move  backwards. 
This  alternate  contraction  of  the  muscles  does  not  occur  the 
whole  length  of  the  body  at  once,  but  sections  -may  contract 
or  relax,  causing  waves  of  contraction  to  extend  from  one  end 
of  the  animal  to  the  other.  This  method  of  locomotion  is  very 
inefficient  for  an  animal  living  on  a  flat  surface,  and  the  earth- 
worm is  only  able  to  move  slowly  upon  the  ground.  In  his 
underground  burrows,  however,  where  the  animal  nearly  fills 
up  the  burrow,  the  method  of  locomotion  is  much  more  efficient 
and  enables  the  animal  to  move  with  considerable  rapidity. 

Alimentary  System. — As  shown  in  Figure  83,  the  alimentary 
canal  consists  of  five  layers.  On  the  very  inside  next  to  the 
cavity  of  the  intestine  is  a  layer  of  epithelial  cells  (Gr.  epi  = 


THE  EARTHWORM 


169 


upon  +  thele  =  nipple),  ep,  which  secrete  the  digestive  fluids 
and  also  aid  in  the  absorption  of  the  food.  Just  outside  of 
these  is  a  layer  of  blood  vessels,  v.  A  third  layer  consists 
of  circular  muscle  fibers  extending  around  the  intestine,  cm, 
and  outside  of  this  is  a  layer  of  longitudinal  muscles,  Im.  A 
fifth  layer  on  the  outside  consists  of  a  thick  coat  of  cells  known 
as  chlorogogen  cells,  c.  These  cover  the  intestine  with  a 
thick  layer  on  its  outer  surface  and  also  form  the  substance 
of  the  typhlosole,  which,  as 
shown  in  Figure  81,  lies 
within  the  cavity  of  the  in- 
testine. The  function  of  the 
chlorogogen  cells  is  not 
known,  though  it  is  probable 
that  they  have  something 
to  do  with  the  absorption  FIG.  83. —  MAGNIFIED  VIEW  OF  A 
of  food  and  possibly  have  a  SECTION  OF  THE  ALIMENTARY  CANAL 
function  of  secretion.  On 
either  side  of  the  oesophagus 
in  the  tenth,  eleventh,  and 
twelfth  segments  are  three 
pairs  of  white  bodies  known  as  calciferous  glands  (Lat.  calx 
=  lime  -\-ferre  =  to  bear),  producing  a  lime  secretion  which 
is  poured  into  the  intestine.  Its  function  is  probably  to  reduce 
the  acidity  of  the  food,  although  very  little  is  known  about 
these  glands  or  their  uses. 

The  Nervous  System. — The  microscopic  study  of  the  nerv- 
ous system  of  the  earthworm,  as  well  as  of  all  higher  animals, 
has  shown  that  while  there  are  several  kinds  of  cells  in  ijb, 
the  chief  ones,  and  probably  the  only  ones  possessing  nervous 
functions,  are  large  cells  called  neurons. 

Neurons. — A  single  neuron  of  the  earthworm  is  shown  in 
Figure  84  A.  It  has  a  rather  irregular  rounded  body,  with  a 
prominent  nucleus,  and  from  it  arises  a  long  process,  much 
longer  than  appears  in  the  figure.  Side  branches  of  this  proc- 


c,  chlorogogen  cells; 

cm,  circular  muscles; 

ep,  epithelium,  lining  the  canal; 

Im,  longitudinal  muscles; 

v,  blood  vessels. 

(Modified  from  Sedgwick  and  Wilson.) 


170 


BIOLOGY 


ess  may  be  seen  near  the  cell  body.  Other  much  shorter 
processes  arise  also  from  the  cell  body  and  divide  quickly 
into  branches.  The  long  fiber  is  called  the  axon  or  the  nerve 
fiber,  and  the  other  branching  projections  are  called  dendrites 
(Gr.  dendron  =  tree).  Sometimes  the  axons  at  their  outer  or 
peripheral  end  break  up  into  numerous  branches  known  as 


A,  a  single  neuron;    B,  a  section  of  the  ventral  surface,  showing  the  nerve 
cord  and  its  connection  with  the  muscles  and  dermis. 

arb,  arborization  of  an  afferent  nerve;  «/,  sensory  nerve  fiber; 

mf,  motor  fiber;  so,  sensory  organ. 

mn,  motor  nerve  cell; 

arborizations  (Lat.  arbor  =  tree),  arb.  In  such  a  neuron  im- 
pulses enter  the  cell  body  through  the  dendrites  and  pass  out 
through  the  axon. 

Similar  neurons  make  up  the  nervous  system  of  all  animals 
which  have  been  carefully  studied.  In  shape  the  neurons  are 
quite  varied  (Fig.  85),  but  in  all  cases  there  is  a  cell  body 
with  one  or  more  branching  processes  arising  from  it;  and  an 
axon  fiber  of  varying  length  extends  outward  from  the  cell. 

Vast  numbers  of  these  neurons  are  aggregated  together  to 
make  the  nervous  system  of  the  earthworm.  The  cerebral  gan-. 
glia  contain  them  in  great  numbers,  and  the  many  nerves  shown 
in  Figure  79  are  formed  chiefly  of  bundles  of  the  axons  of  the 
neurons,  whose  cell  bodies  are  either  in  the  ganglia  or  at  the 


THE  EARTHWORM 


171 


outer  ends  of  the  nerves,  in  the   prostomial   lobe,  etc.     The 

ventral  cord  also  is  a  mass  of  neurons,  and  since  it  is  simpler 

than  the    brain   it 

may  be  more  easily 

understood  and  will 

illustrate  better  the 

relation  of  neurons 

to  the  rest  of  the 

body. 

The  ventral  cord. 

— A  cross  section  of 

the  cord   shows  it 

to    be   covered   on 

the   outside    by   a 

thin  layer  of  epi- 
thelium, the  perito- 
neum, inside  of 

which  is  a  muscular 

sheet  containing  a 

few  blood   vessels; 

Fig.  86.    Near  the  dorsal  surface  of  the  cord  are  three  clear  rods 

running  lengthwise, 
called  giant  fibers, 
g,  containing  nerve 
fibers.  The  cord 
itself  is  really  two 
cords  fused  to- 
gether. Embedded 
in  the  cord  may  be 
seen  many  large 
cells,  which  are  the 
bodies  of  the  neu- 
rons making  up  the 

cord;  and  extending  out  to  form  the  nerve  fibers  which  arise 

from  the  cord  are  the  axons  of  these  neurons. 


FIG.  85. —  NEURONS   OF  VARIOUS  TYPES 

FROM   HIGHER    ANIMALS 

A,  a  complex  of  neurons  from  the  cerebrum;  B  and  C, 
neurons  from  the  cerebellum;  D,  a  single  neuron  from  the 
cerebrum. 


n 


FIG.  86. —  MAGNIFIED  SECTION  OF  THE  VENTRAL 
CORD  OF  THE  EARTHWORM 


g,  giant  fibers; 
n,  neurons; 


nf,  nerve  fibers; 
v,  blood  vessels. 


172  BIOLOGY 

The  relation  of  these  neurons  to  the  body  may  be  seen  from 
Figure  84.  Most  of  the  cells  which  appear  so  prominently 
in  the  cord  have  connections  as  shown  at  mn.  Each  has  a 
complex  of  dendrites  which  branch  in,  the  substance  of  the 
cord,  and  a  single  axon  which  passes  out  through  the  nerve 
to  be  finally  distributed  to  the  muscles.  These  neurons  send 
impulses  to  the  muscles  and  are  called  motor  cells.  Some  send 
their  axons  to  muscles  on  the  same  side,  as  shown  in  the  figure, 
and  others  send  theirs  across  the  cord  to  the  muscles  on  the 
other  side  of  the  body.  These  axons  are  known  as  efferent 
(Lat.  ex  =  out  +  ferre  —  to  bear)  nerve  fibers.  Some  of  the 
axons  do  not  pass  out  of  the  cord,  but  simply  connect  dif- 
ferent parts  of  the  cord  itself. 

The  neurons  which  carry  impulses  from  without  toward  the 
center  are  called  afferent  (Lat.  ad  =  to  +  ferre  =  to  bear) 
neurons.  These  never  have  their  neuron  bodies  within  the 
cord  but  somewhere  outside  it.  Many  of  them  take  their 
origin  in  special  cells  called  sense  cells  (Fig.  84  so),  which  are 
sensitive  to  certain  external  stimuli.  The  impulses  excited  in 
the  cell  pass  over  the  axon  to  the  ventral  cord.  Where  the 
axon  enters  the  cord  it  breaks  up  into  numerous  branches,  or 
arborizations,  arb,  which  spread  out  in  the  cord  itself.  The 
impulses  entering  by  the  axons  may  pass  from  the  arboriza- 
tions to  the  dendrites  of  the  motor  cells  and  excite  them  to 
action.  Hence  a  stimulus  applied  to  the  skin  may  produce  a 
movement. 

The  sense  organs. — The  cells  at  the  end  of  the  afferent 
nerves  constitute  the  sense  organs,  and  they  are  so  constructed 
as  to  be  influenced  by  different  external  forces.  The  earthworm 
has  no  eyes,  although  some  of  its  sense  organs  appear  to  be 
slightly  affected  by  a  bright  light.  They  have  no  ears  and  no 
sense  of  sound,  though  they  are  very  sensitive  to  a  slight  jar. 
They  have  a  sense  of  taste,  located  in  the  mouth,  and  also  a 
sense  of  smell.  None  of  the  sense  organs  is  visible  to  the  naked 
eye,  but  they  may  be  seen  by  microscopic  study.  The  end  of  the 


THE  EARTHWORM  173 

prostomial  lobe  is  the  most  sensitive  part  of  the  body;  here 
the  sense  cells  are  most  abundant  and  here  the  nerve  supply  is 
the  largest;  Fig.  79. 


LABORATORY  WORK  ON  THE  EARTHWORM 

Only  large  specimens  should  be  used.  These  can  be  purchased  from 
dealers  in  natural  history  supplies  or  they  may  be  collected  by  searching 
with  a  lantern  on  a  dark  night,  when  they  may  be  found  stretched  out 
on  the  ground  and  thus  readily  collected.  A  little  care  and  experience 
is  needed  to  do  this  without  disturbing  them,  for  they  are  very  sensitive 
to  the  slightest  jar  and  quickly  retreat  into  their  burrows. 

The  specimens  should  first  be  studied  alive,  if  possible,  to  see  the  con- 
traction of  the  dorsal  blood  vessel  and  the  contractions  of  the  body  in 
locomotion.  The  setae  may  be  felt  by  drawing  the  body  gently  through 
the  fingers,  and  they  can  be  examined  under  a  lens. 

If  the  worms  are  to  be  dissected,  or  preserved  for  future  use,  they  should 
be  treated  as  follows:  Place  the  worms  in  a  shallow  dish  with  wet  filter 
paper  torn  into  shreds.  The  animals  will  swallow  it  and  as  it  passes 
through  the  alimentary  canal  it  will  carry  the  dirt  from  the  canal.  This 
part  of  the  process  is  not  necessary  unless  microscopic  sections  are  to  be 
made.  If  they  are  to  be  kept  simply  for  dissection,  they  can  be  preserved 
at  once  as  follows:  — • 

Place  a  number  of  worms  in  a  shallow  dish  with  just  water  enough 
to  cover  them.  Add  a  few  drops  of  alcohol,  and,  after  a  few  moments, 
add  a  little  more.  Continue  adding  the  alcohol  gradually  until  the  ani- 
mals have  become  motionless  and  relaxed.  This  process  should  take  at 
least  two  hours.  Then  transfer  them  to  a  large  shallow  dish  containing 
50%  alcohol,  straightening  the  animals  out,  and  laying  them  side  by  side. 
After  an  hour  replace  the  50%  alcohol  with  70%;  after  a  few  hours  change 
again  to  a  fresh  lot  of  70%  alcohol.  Finally  the  animals  are  to  be  placed 
in  90%  alcohol.  It  is  important  to  keep  them  straight  in  this  final  hard- 
ening fluid,  and  this  may  be  done  by  laying  them  out  on  rather  stiff  paper, 
without  touching  each  other,  and  rolling  them,  putting  about  a  dozen  in 
each  roll.  This  will  hold  them  in  proper  shape,  and  the  rolls  may  be 
stored  in  tall  jars  and  will  keep  indefinitely. 

Animals  so  preserved  will  serve  either  for  microscopic  sections  or  for 
dissection.  Sections  should  be  made  by  the  instructor  and,  after  stain- 
ing, should  be  mounted  and  furnished  the  student  for  study. 

For  dissection,  the  animal  should  be  placed,  under  water,  in  a  tray 
containing  dissecting  wax.  The  anterior  end  is  pinned  down  and  then, 


174  BIOLOGY 

with  fine  scissors,  an  incision  is  made  along  the  dorsal  median  line,  from 
the  head  to  the  posterior  end  of  the  body.  The  body  is  then  opened  and 
the  walk  pinned  out  so  as  to  disclose  the  internal  parts.  This  should  all 
be  done  under  water.  If  carefully  performed  the  internal  parts  may  be 
easily  worked  out,  a  lens  being  used  to  show  the  smaller  parts.  To  show 
the  nervous  system  and  the  nephridia  the  alimentary  canal  should  be 
cut  through,  behind  the  gizzard,  and  carefully  dissected  away  in  front. 
There  will  then  be  no  difficulty  in  making  out  all  the  organs  except  the 
ovaries  and  spermaries.  The  ovaries  may  be  found  by  careful  study 
with  a  lens,  but  the  spermaries  cannot  be  found  without  special  methods. 
The  contents  of  the  seminal  vesicles  and  the  ovaries  should  be  examined 
with  a  microscope.  One  of  the  nephridia  should  be  removed  and  studied 
with  a  low  magnifying  power. 

For  the  study  of  the  histology,  sections  should  be  furnished  by  the  in- 
structor. Animals  preserved  as  above  described  are  in  good  condition 
for  sectioning.  They  should  be  embedded  in  paraffin  and  stained  in  picro- 
carmine.  Sections  through  various  parts  of  the  body  should  be  studied, 
and  these  should  include  at  least  sections  through  the  cerebral  ganglia, 
through  the  aortic  arches,  and  through  the  posterior  parts  of  the  body  show- 
ing the  typhlosole.  The  study  of  these  sections  with  both  low  and  high 
powers  will  show  the  chief  features  of  the  microscopic  anatomy.  More 
detailed  study  of  the  histology  is  hardly  feasible  with  elementary  classes. 


CHAPTER  IX 

MULTICELLULAR  ANIMALS:  THE  FROG  (RANA) 
GENERAL  DESCRIPTION 

THE  body  of  the  frog  is  composed  of  a  head  and  a  trunk, 
but  there  is  neither  neck  nor  tail.  The  wide  mouth  extends 
far  back  to  the  end  of  the  head.  On  the  upper  side  of  the  head 
in  front  are  two  nostrils  (nares)  that  open  directly  through 
the  bones  of  the  skull  into  the  mouth.  Farther  back  on  either 
side  of  the  head  are  the  eyes,  provided  with  two  loose  folds 
of  skin  which  serve  as  eyelids.  The  upper  lid  is  immovable, 
but  the  lower  can  be  brought  up  over  the  eye  for  protection. 
It  is  called  the  nictitating  membrane  (Lat.  nictare  =  to  wink), 
is  semi-transparent,  and  does  not  prevent  sight  wholly  when 
closed.  Behind  the  eyes  are  two  round  flat  surfaces,  which 
are  membranes  stretched  over  a  shallow  cavity  in  the  skull. 
They  are  the  tympanic  membranes  (Lat.  tympanum  =  drum) 
and  serve  to  collect  sound  waves  and  transfer  them  to  the 
ears  which  lie  within  the  head.  The  part  of  the  body  behind 
the  anterior  appendages  or  arms  is  called  the  abdomen,  and 
the  cavity  within,  which  holds  the  stomach  and  intestines, 
is  the  abdominal  cavity.  The  organs  of  the  abdomen  are 
sometimes  called  viscera. 

Of  the  two  pairs  of  appendages,  the  fore  legs  are  provided 
with  only  four  toes,  while  the  hind  legs  have  five  toes  con- 
nected by  a  web.  The  hind  legs  are  much  longer  than  the 
fore  legs  and  are  the  chief  organs  used  in  locomotion.  The 
rest  of  the  body  is  smooth,  gradually  tapering  behind  and  end- 
ing abruptly  just  above  the  attachment  of  the  hind  legs.  Near 
the  posterior  end  of  the  body  on  the  dorsal  side  is  a  good-sized 
opening,  the  cloacal  aperture  (Lat.  cloaca  —  sewer),  which 
serves  as  the  common  outlet  of  the  intestine,  the  kidneys, 
and  the  reproductive  organs. 

175 


176 


BIOLOGY 


The  whole  body  of  the  frog  is  covered  with  a  smooth  skin, 
which  is  always  moist  and  is  abundantly  supplied  with  blood 
vessels,  especially  under  the  arms  and  on  the  side  of  the  body. 
The  skin  is  everywhere  loosely  attached  to  the  underlying 
flesh  and  in  certain  rather  large  areas  is  not  attached  at  all, 
large  spaces  being  thus  left  between 
it  and  the  flesh.  These  are  lymph 
spaces  and  are  filled  with  a  clear  liquid 
called  lymph.  When  the  skin  is  ex- 
amined microscopically,  it  is  found  to 
be  made  of  two  layers;  Fig.  87.  The 
outer  layer,  the  epidermis,  ep,  is  thin, 
while  the  inner  layer,  the  dermis,  d,  is 
quite  thick.  The  epidermis  is  made 
of  several  layers;  the  cells  of  the  inner 
layers  are  large,  rounded,  growing  cells, 
while  the  outer  ones  are  flattened  and 
lifeless.  The  epidermis  increases  in 
thickness  from  its  inner  side,  and  is 
constantly  wearing  away  on  its  outer 
side.  The  dermis  is  a  mass  of  con- 
nective tissue  fibers,  among  which  lie 
glands,  blood  vessels,  nerves,  and 

numerous  pigment  (Lat.  pingere  =  to  paint)  cells  which  give 
the  color  to  the  skin. 

The  Skeleton. — The  frog  has  an  internal  bony  skeleton. 
An  internal  skeleton  is  the  most  distinctive  characteristic 
of  the  highest  animals.  Animals  with  such  a  skeleton  are 
called  vertebrates,  a  group  comprising  fishes,  amphibians, 
reptiles,  birds,  and  mammals.  No  other  animals  except  verte- 
brates possess  true  bones.  This  bony  skeleton  gives  support 
to  the  softer  parts,  gives  form  to  the  body,  serves  to  attach 
the  muscles,  and  enables  them  to  produce  the  movements  of  the 
animal.  The  skeleton  is  made  of  about  ninety  articulated  bones, 
i.  e.t  united  together  at  the  joints.  Some  of  these  form  mov- 


FIG.   87.  —  SECTION 
THROUGH  THE  SKIN  OF 

THE   FROG 

ep,  epidermis; 
d,  dermis. 

(Modified  from  Howes.) 


THE  FROG  177 

able  joints,  in  which  a  movement  of  the  bones  produces  a 
movement  of  the  body.  In  other  joints  the  bones  are  firmly 
grown  together  forming  the  immovable  joints.  The  bones  of 
the  skull,  for  example,  are  so  firmly  fused  that  they 
appear  as  a  single  bone;  and  the  bone  of  the  forearm  (Fig. 
88  r-u)  is  really  made  of  two  bones  fused  together.  Two  dis- 
tinct parts  of  the  skeleton  may  clearly  be  seen:  (I). the  axial 
skeleton,  consisting  of  the  skull  and  spinal  column;  (2)  the  ap- 
pendicular  skeleton,  which  forms  the  support  for  the  arms 
and  legs. 

The  axial  skeleton. — The  spinal  column  is  composed  of 
nine  separate  bones  called  vertebrae;  Fig.  88  B.  Each  ver- 
tebra consists  of  9  centrum,  c,  and  a  neural  arch,  na,  the 
arch  inclosing  the  neural  foramen  (Lat.  foramen  =  opening). 
From  each  side  of  the  arch  a  process  of  bone  extends  laterally, 
called  the  transverse  process  (Lat.  trans  =  across  -f  vertere  = 
to  turn) ,  tr.  On  the  front  and  back  of  each  vertebra  are  two 
smooth  surfaces  where  the  successive  vertebrae  rest  upon 
each  other,  i.  e.,  articulate  (Lat.  articulus  =  joint).  They  are 
the  articular  processes,  or  zygapophyses.  In  their  natural 
position  the  nine  vertebrae  are  joined  together  by  their  centra, 
the  posterior  surface  of  one  touching  the  anterior  surface 
of  the  next;  Fig.  A.  The  neural  foramina  are  thus  placed 
opposite  each  other,  and  all  together  form  a  tube  which  in- 
closes the  spinal  cord.  The  surfaces  of  the  centra  fit  by  a 
ball-and-socket  joint,  each  of  the  first  seven  vertebrae  having 
a  ball  on  the  posterior  and  a  socket  on  the  anterior  surface, 
while  the  eighth  is  concave  on  both  surfaces,  and  the  ninth 
is  convex  on  both  surfaces.  The  nine  vertebrae  are  much 
alike,  but  can  be  distinguished  from  each  other.  The  first 
has  no  transverse  process,  while  the  centrum  of  the  ninth  has 
two  convex  posterior  surfaces,  and  very  large  transverse  proc- 
esses. From  the  posterior  surface  of  the  last  vertebra  a  long 
slender  bone  extends  backward  to  the  end  of  the  body,  the 
urostyle  (Gr.  oura  —  tail  +  stylos  =  pillar);  Fig.  A,  ur.  The 


178 


BIOLOGY 


FIG.  88. — THE  SKELETON  OP  THE  FROG 


THE  FROG  179 


FIG.  88. —  THE  SKELETON  OF  THE  FROG 

A,  one-half  of  the  skeleton  shown  from  above. 

as,  astragalus;  mt,  metatarsals; 

c,  carpals;  mx,  maxilla; 

ca,  calcaneum;  na,  nasal; 

cr,  cms;  o,  opening  for  nerve; 

e,  ethmoid;  p,  parietal; 

ex,  exoccipital;  ph,  phalanges; 

/,  frontal;  pr,  premaxilla; 

fe,  femur;  r-u,  radio-ulnar; 

fm,  foramen  magnum;  sq,  squamosal; 

it,  ilium;  t,  tarsals; 

hu,  humerus;  tr,  transverse  process; 

me,  metacarpals;  ur,  urostyle. 

B,  a  vertebra  from  the  end  and  from  above. 

c,  centrum; 

na,  neural  arch; 

tr,  transverse  process. 

C,  the  skull  shown  from  below. 

con,  occipital  condyle;  pt,  pterygoid; 

«,  Quadrate; 
*>.  vomer. 

D,  skull  shown  from  the  side. 

ex,  exoccipital;  g,  quadrato-jugal; 

mx,  maxilla;  qu,  quadrate; 

m,  mandible;  sq,  squamosal. 
na,  nasal; 

E,  the  hyoid  bone. 

F ,  the  shoulder  girdle  shown  from  below. 

co,  coracoid;  pr,  precoracoid; 

gc,  glenoid  cavity;  sc,  scapula; 

h,  humerus;  st,  sternum. 
ost,  omosternum; 

(of,  the  pelvic  girdle  shown  from  the  side. 

ac,  acetabulum;  is,  ischium; 

il,  ilium;  pu,  pubis. 


180  BIOLOGY 

spinal  cord  extends  into  it,  but  soon  passes  out  through  two 
small  openings,  on  either  side,  o,  as  two  small  filaments.  This 
bone  represents  the  tail  found  in  allied  animals  (salamanders). 
The  frog  has  no  ribs  and  the  transverse  processes  end  abruptly 
at  a  short  distance  from  the  centrum. 

The  skull. — In  front  the  first  vertebra  is  articulated  with 
the  skull,  and  the  neural  canal  is  continued  into  the  skull 
through  a  large  opening,  called  the  foramen  magnum  (Lat. 
foramen  =  hole),  fm.  Inside  the  skull  is  a  large  cavity  hold- 
ing the  brain,  the  cranial  cavity.  The  skull  itself  is  com- 
posed of  thirty-two  bones,  rigidly  fused  together  to  form  a 
solid  structure.  These  bones,  which  are  shown  and  named 
in  Figure  88  A  and  C,  may  be  divided  into  three  groups :  1 .  The 
cranial  bones,  which  form  the  roof,  walls,  and  floor  of  the  cranial 
cavity.  The  floor  is  made  of  the  basioccipital  and  the  para- 
sphenoid,  ps;  the  walls  are  made  of  the  parietals,  p,  the  otic 
bones,  and  the  exoccipitals,  ex;  and  the  roof  is  made  of  the 
supraoccipitals  and  the  frontals,  /.  2.  The  facial  bones,  which 
form  the  face.  These  are  the  nasals,  na,  the  premaxillas,  pr, 
and  the  maxillas,  mx,  above,  and  the  vomers,  vo,  below.  3.  The 
branchial  (Lat.  branchice  =  gills)  skeleton.  This  part  of  the 
skeleton  is  made  primarily  of  two  V-shaped  arches,  lying  below 
the  cranium  with  the  open  part  of  the  V  above,  next  to  the  skull ; 
but  the  original  relation  of  the  V-shaped  arches  has  become  so 
modified  that  it  is  difficult  to  recognize.  The  first  of  the  arches 
is  the  lower  jaw  or  mandible;  Fig.  Z>,  m.  The  closed  part 
of  this  arch  is  in  front  where  the  two  halves  come  together. 
At  the  back  the  two  halves  spread  apart  and  pass  backward 
to  the  point  where  the  jaw  articulates  with  the  cranium  at  q. 
The  lower  jaw  is  from  this  joint  held  attached  to  the  cranium 
by  two  chains  of  bones.  One  of  them  is  made  of  the  quadrate 
(Fig.  D,  qu),  and  the  squamosal,  sq,  these  two  forming  what 
is  sometimes  called  the  suspensorium.  The  other  chain  is 
made  of  two  bones  lying  below  the  cranium,  the  pterygoid 
(Fig  C,  pi),  and  the  palatine,  pa.  These  are  firmly  fixed  to 


THE  FROG  181 

the  cranium  below.  The  joint  is  also  attached  to  the  max- 
illa by  a  little  bone  called  the  quadrato-jugal ;  Fig.  D,  q. 
Although  in  the  adult  frog  these  chains  of  bones  are  firmly 
attached  to  the  cranium,  they  are  at  first  free  from  it,  and  are 
really  the  upper  parts  of  the  arches  below,  rather  than  a  part 
of,  the  cranium  proper.  The  second  arch  is  very  rudimentary, 
only  a  small  part  of  it  being  left  in  the  frog.  It  is  called  the 
hyoid  arch.  Although  in  some  animals  this  is  also  a  well- 
developed  V-shaped  arch,  all  that  is  left  of  it  in  the  frog  is  a 
flat  plate,  made  partly  of  bone  and  partly  of  cartilage  (Fig. 
E),  which  is  so  loosely  attached  to  the  skull  that  it  is  usually 
lost  in  prepared  skulls.  In  the  living  frog  it  lies  underneath 
the  larynx,  to  which  it  gives  support  and  rigidity.  It  is  attached 
to  the  skull  only  by  ligaments,  without  any  bony  connection. 

When  the  skull  begins  to  form  in  the  young  frog  the  parts 
are  soft,  and  only,  as  development  proceeds,  does  true  bone 
form.  Part  of  the  skull  forms  first  as  cartilage,  a  material 
that  is  harder  than  membrane  but  softer  than  bone.  Later 
within  this  cartilage  the  mineral  matter  is  deposited,  forming 
true  bone,  and  the  bones  thus  formed  are  consequently  called 
cartilage  bones.  These  are  the  octipitals,  palatines,  pterygoids, 
and  the  mandibles.  The  other  bones  are  formed  first  as  mem- 
branes rather  than  cartilage.  Within  the  membrane  the  mineral 
bony  matter  is  laid  down,  and  bones  developing  in  this  manner 
are  known  as  membrane  bones.  The  membrane  bones  are 
ihefrontals,  parietals,  parasphenoids,  squamosals,  nasals,  vomers, 
premaxilla,  and  the  maxilla. 

At  its  posterior  end  the  skull  is  articulated  with  the  first 
vertebra  by  means  of  two  rounded,  smooth  surfaces  which  fit 
into  two  corresponding  smooth  depressions  on  the  upper  sur- 
face of  the  first  vertebra.  The  articular  projections  are  called 
the  occipital  condyles;  Fig.  C,  con. 

Appendicular  skeleton. — Each  appendage  consists  of  a  girdle 
and  the  appendage  proper.  The  shoulder  girdle  is  a  girdle 
of  bones  surrounding  the  body  just  back  of  the  head,  and 


182  BIOLOGY 

holding  the  arm  in  position.  It  is  shown  from  below  and 
flattened  out  in  Figure  F.  Each  half  consists  of  a  scapula,  sc 
(the  dorsal  part  of  which  is  made  of  cartilage),  a  coracoid, 
co,  a  precoracoid  and  a  clavicle  fused  together,  pr.  At  the 
place  where  the  coracoid  and  the  scapula  come  together  is  a 
smooth  cavity  into  which  the  end  of  the  arm  articulates,  called 
the  glenoid  cavity,  gc.  In  its  natural  position  the  scapula 
is  bent  over  the  back,  with  the  coracoids  touching  each  other 
in  the  middle  line  below  on  the  ventral  side  of  the  body.  Be- 
hind and  in  front  of  them  are  two  pieces  of  bone,  the  omo- 
sternum,  ost,  and  the  sternum,  st.  These  two  bones  are  re- 
garded as  a  part  of  the  axial  skeleton. 

The  arm  proper  consists  of  the  humerus  (Fig.  A,  hu), 
the  radius  and  ulna  fused  together,  r-u,  six  wrist  or  carpal 
bones,  c,  and  five  fingers,  of  which  the  first  is  rudimentary. 
Each  finger  is  composed  of  a  metacarpal,  me,  and  several 
phalanges,  ph.  The  posterior  appendages  have  a  pelvic  girdle, 
made  of  three  pairs  of  bones,  all  united  into  one  in  the  adult. 
One  of  them,  the  ilium,  is  long  and  runs  forward  to  the  trans- 
verse process  of  the  last  vertebra;  Fig.  A,  il.  At  its  posterior 
end  each  ilium  joins  the  other  two  bones,  the  pubis  (Fig.  G, 
pu),  and  the  ischium,  is.  At  the  point  where  the  three  bones 
meet  there  is  a  rounded  cavity  for  the  attachment  of  the  leg, 
the  acetabulum,  ac.  The  pubes  and  ischia  of  the  two  sides 
of  the  body  are  fused  together  on  the  middle  line,  below  the 
urostyle.  The  leg  consists  of  a  femur  (Fig.  A,  fe),  and  the 
cms,  cr,  which  is  really  composed  of  a  tibia  and  fibula  fused 
together.  Following  the  crus  are  the  bones  of  the  foot,  consist- 
ing of  two  slender  bones,  the  astragalus,  as,  and  calcaneum,  ca; 
then  come  two  extremely  small  tarsal  bones,  t,  and  finally 
a  series  of  metatarsals,  mt,  and  phalanges,  ph. 

Muscular  System. — Most  of  the  bones  of  the  skeleton  are 
more  or  less  movable  one  upon  the  other  at  the  articulations. 
The  muscles  which  move  them  are  numerous  and  complicated. 
Each  muscle  is  an  elongated  mass  of  contractile  tissue,  which 


THE  FROG 


183 


ab,  adductor  brevis; 
al,  adductor  longus; 
am,  adductor  magnus; 
d,  deltoid; 
ec,  extensor  cruria; 
gc,  gastrocnemius; 


FIG.  89. —  THE  MUSCLES  OF  THE 

FROG   FROM   BELOW,    THE   SKIN 
BEING   REMOVED 

The    more    important     muscles    are 
named  as  follows:  — 
o,  obliqus; 
p,  pectoralis; 
r,  rectus  abdominis; 
ri,  rectus  internus  major; 
a,  submaxillaris; 


st,  sartorius: 

t,  triceps; 

to,  tibialis  anticus; 

tp,  tibialis  posticua; 

vi,  vast»s  ipt«rnus. 


184  BIOLOGY 

is  usually  attached  at  the  ends  to  two  separate  bones,  the 
term  origin  being  applied  to  the  attachment  nearest  to  the 
center  of  the  body,  and  insertion  to  the  attachment  the  farthest 
from  the  center;  muscles  pull  in  the  direction  of  their  origin. 
Since  these  muscles  are  numerous  and  attached  to  the  bones 
at  different  places,  they  pull  upon  the  bones  in  different  direc- 
tions and  produce  a  great  variety  of  movements.  Figure  89 
shows  the  chief  muscles  of  the  frog.  The  names  given  to  them 
are  the  same  as  those  applied  to  the  corresponding  muscles 
in  man. 

Joints  or  Articulations. — Where  two  bones  come  together 
they  form  a  joint.  In  some  cases  the  bones  are  so  rigidly  grown 
together  that  there  is  no  motion  between  them,  thus  forming 
the  fixed  joints,  like  those  that  are  between  the  bones  which 
form  the  skull.  In  other  places  the  bones  are  freely  movable, 
forming  the  movable  joints.  All  the  movements  of  the  body 
are  produced  at  the  joints.  The  bones  at  these  joints  are  so 
connected  that,  while  they  are  held  firmly  together,  they  are 
at  the  same  time  freely  movable.  The  ends  of  the  bones  are 
generally  more  or  less  rounded,  the  end  of  one  bone  fitting 
into  a  rounded  depression  on  the  other.  The  ends  of  the  bones 
are  also  covered  by  a  layer  of  cartilage,  which  is  quite  smooth 
so  as  to  prevent  friction.  This  structure  makes  it  possible 
for  one  bone  to  move  upon  the  other  without  difficulty.  All 
friction  is  eliminated,  and  movement  of  the  bones  is  rendered 
easier,  by  a  secretion  of  fluid  which  is  poured  into  the  joint 
from  the  synovial  glands.  This  is  called  the  synovial  fluid. 
To  prevent  the  bones  from  being  pulled  apart  they  are  held 
together  by  bands  of  white  connective  tissue  called  ligaments. 
These  are  tough  but  flexible,  and  are  attached  to  the  two  bones 
that  form  the  joint.  They  are  long  enough  to  make  the  mo- 
tions of  the  bones  free,  but  short  enough  to  hold  them  in  posi- 
tion and  prevent  their  being  pulled  away  from  each  other  by 
slight  strains.  The  bones  are  held  firmly  in  position  by  the 
muscles.  The  muscles  which  move  the  bones  usually  have 


THE  FROG  185 

their  origin  on  the  bones  above  the  joint,  and  their  insertion 
on  the  bone  below.  The  muscles  end  in  bands  of  connective 
tissue  called  tendons  that  extend  down  over  the  joint  to  the 
insertion  on  the  lower  bone.  The  muscles  are  always  tightly 
stretched  in  the  body  and  always  pulling  upon  the  tendons. 
As  a  result  the  tension  upon  the  tendons  holds  the  two  bones 
of  the  joint  in  firm  contact.  Outside  of  the  muscles  and  tendons 
is  the  skin.  The  joint  thus  consists  of  smoothly  moving  bones, 
which  are  moistened  by  synovial'  fluid,  held  in  position  by 
tightly  drawn  tendons,  prevented  from  being  pulled  apart  by 
ligaments  that  protect  them  against  strains,  and  moved  by 
muscles. 

The  freedom  of  motion  in  the  different  joints  varies  with 
the  shape  of  the  bones  at  the  joints.  In  some  of  the  articula- 
tions, one  bone  ends  in  a  ball  which  fits  into  a  rounded  socket 
of  the  other  bone.  In  this  type,  the  ball-and-socket  joint, 
the  bones  are  freely  moved  in  any  direction.  The  joint  at 
the  hip  and  that  of  the  shoulder  are  examples  of  this  type. 
In  other  joints  the  form  of  the  bones  is  such  that  motion  is 
possible  only  backward  or  forward.  These  are  called  hinge 
joints,  and  are  illustrated  by  the  joints  at  the  elbow,  the  knee, 
the  wrist,  and  by  the  separate  joints  of  the  fingers  and  toes. 
In  some  joints  one  bone  moves  around  the  other  as  on  a  pivot. 
No  good  examples  of  this  are  found  in  the  frog,  but  in  the  human 
body  the  motion  of  turning  the  head,  or  turning  over  the  hand 
so  that  the  back  or  the  palm  is  uppermost,  are  excellent  illus- 
trations. It  is  evident  that  the  movements  of  the  body  are 
dependent  upon  the  free  motion  of  the  bones  at  the  joints, 
and  that  the  growing  of  the  bones  together  at  a  joint,  anchy- 
losis as  it  is  called,  will  destroy  all  power  of  motion. 

Alimentary  Canal. — The  wide  mouth  (oral  opening)  leads 
into  a  very  large  cavity,  the  buccal  cavity.  There  are  teeth 
on  the  maxilla,. premaxilla,  and  vomer  (Fig.  88  C,  D),  which 
are  of  use  for  holding,  but  not  for  masticating  food.  On  the 
floor  of  the  mouth  is,  a  large  muscular  tongue,  attached  to 


186 


BIOLOGY 


.oe 


the  base  of  the  mouth  in  front,  and  free  behind.  Owing  to 
this  attachment,  the  back  part  of  the  tongue  can  be  thrown 
out  of  the  mouth  for  a  considerable  distance,  serving  as  an 
important  organ  for  capturing  insects.  Just  back  of  the  tongue 
on  the  floor  of  the  mouth  is  a  narrow  slit,  the  glottis,  leading 
into  a  tube,  which  passes  to  the  lungs.  Behind  the  glottis 

is  a  larger  opening  leading  to  the 
oesophagus,  and  hance  to  the  stom- 
ach. The  nostrils  open  in  the  mouth 
through  the  roof  in  front  (internal 
nares);  and  a  pair  of  openings  in  the 
back  part  of  the  roof  leads  to  the  ears, 
the  eustachian  openings. 

If  a  slit  be  made  through  the  skin 
and  flesh  of  the  abdomen,  passing 
forward  on  the  ventral  middle  line 
through  the  sternum,  and  the  body 
opened,  most  of  the  internal  organs 
can  be  seen.  The  oesophagus  passes 
directly  backward  about  halfway  to 
the  end  of  the  body,  where  it  ex- 
pands into  a  large  chamber,  the 
stomach  (Fig.  90  st),  which  extends 
obliquely  across  the  body  towards 
the  right.  The  lower  part  of  the 
l-»  d  stomach  is  called  the  pylorus ;  this 
passes  down  into  a  small  tube  which 
FIG.  90.— THE  ALIMENTARY  makeg  a  u_shaped  bend,  called  the 

TRACT   OF  THE   FROG  ^fo  4       and     then     formg      gey_ 

blt  bladder;  oe,  oesophagus; 

el,  cloacal  cavity;    p,  pancreas;  eral    COllS    Which    Constitute    the     UltCS- 

d,  duodenum;  sp,  spleen;  . 

gb,  gall  bladder;       at,  stomach;  11116,  in.       r  inally  the  tube  paSSCS  into 

in,  intestine;  r,  rectum. 

a  large  but  short  chamber,  the  rectum, 

r,  which  communicates  with  the  exterior  through  the  cloacal 
opening.  In  front  of  and  to  the  right  of  the  stomach  is  the  large 
several-lobed  liver.  This  secretes  a  liquid  called  bile,  which 


THE  FROG 


187 


passes  by  a  duct  into  the  gall  bladder,  gb,  where  it  is  stored 
for  a  while  and  from  which  it  later  passes  through  the  bile 
duct  into  the  duodenum,  close  to  the  pyloric  end  of  the  stomach. 
In  the  bend  of  the  U  formed  by  the  duodenum  and  the  stomach, 
is  a  slender,  yellowish  body,  the  pancreas,  p,  which  empties  into 
the  duodenum  by  the  pancreatic  duct  opening  close  to  the 
bile  duct.  The  lining  of  the  whole  alimentary  canal  is  called 
the  mucous  membrane. 

The  whole  intestine  is  slung  in  position  by  a  thin  sheet  of 
membrane,  which  passes  around  the  intestine  and  then  be- 
comes attached  to  the  abdominal  wall.  This  is  the  mesentery, 
and  is  really  a  fold  of  a  large  membrane  that  completely 
lines  the  body  cavity,  the  peritoneum. 
The  relations  of  the  peritoneum,  mes- 
entery, and  intestine  are  shown  dia- 
grammatically  in  Figure  91.  In  the 
mesentery  are  many  nerves  and  nu- 
merous blood  vessels  which  carry 
nutrition  from  the  intestine.  The 
mesentery  surrounds  not  only  the  in- 
testine, but  also  the  liver  and  the 
pancreas.  In  its  folds  below  the 
stomach  is  a  rounded  red  body,  the 
spleen;  Fig.  90  sp. 

Circulatory  System. — The  circula- 
tory system  of  the  frog,  like  that  of 
the  earthworm,  consists  of  blood  in- 
closed in  a  network  of  blood  vessels;  but  it  is  a  more  definite 
system  and  the  blood  flows  in  a  more  regular  course.  It  con- 
sists of  a  true  heart,  and  blood  vessels. 

The  heart  is  situated  beneath  the  shoulder  girdle,  in  front 
of  the  liver  and  is  surrounded  by  a  thin  sac,  the  pericardium 
(Gr.  peri  =  around  +  cardia  =  heart).  The  heart  itself  is 
made  up  of  a  sac  divided  into  three  chambers,  the  walls  of 
which  are  masses  of  muscles.  The  fibers  of  these  muscles  run 


FIG.  91. —  DIAGRAM  REP- 
RESENTING A  CROSS  SEC- 
TION OF  THE  BODY 

bw,  body  wall; 
in,  intestine; 
mes,  mesentery; 
per,  peritoneum. 


188  BIOLOGY 

in  every  direction,  so  that  when  they  contract  (systole)  the 
heart  is  diminished  in  size  and  the  blood  that  is  in  the  cavities 
is  squeezed  out;  when  they  relax  (diastole)  the  heart  expands 
again  and  the  blood  flows  into  it.  Figure  92  shows  a  diagram 
of  the  heart  structure,  cut  open  so  as  to  show  the  interior  of 
the  cavities.  At  the  anterior  end  are  two  cavities,  the  right 
and  the  left  auricles,  ra  and  la;  the  right,  which  receives  blood 
from  the  body,  being  much  larger  than  the  left,  which  receives 
blood  from  the  lungs.  These  two  chambers  are  separated 
by  a  partition.  At  the  lower  side  of  the  auricles  each  opens 
into  the  ventricle,  v,  the  third  and  largest  chamber  below. 
The  openings  between  the  auricles  and  ventricle  (shown  by  the 
arrows  in  Fig.  102),  are  guarded  by  valves,  which  are  flaps  of 
membrane,  so  situated  that  they  allow  blood  to  flow  readily 
from  the  auricle  into  the  ventricle,  but  close  up  at  once  if  the 
blood  starts  to  flow  back  into  the  auricle,  as  it  would  do  when 
the  ventricle  contracts,  did  not  these  valves  block  the  passage. 
The  ventricle  is  a  large  chamber,  partly  divided  by  partitions. 
Leading  out  of  it  in  front  is  a  large  blood  vessel.  This 
extends  forward,  on  the  ventral  side  of  the  heart,  and 
at  the  anterior  end  of  the  heart  it  divides  into  two  arteries, 
one  turning  to  the  right  and  one  to  the  left.  The  large 
vessel  is  called  the  bulbus  arteriosus,  ba,  and  its  two 
branches  are  the  aortae,  ad.  This  bulbus  receives  the  blood  which 
is  forced  out  of  the  heart  when  it  contracts.  Within  it,  and 
at  the  beginning  of  the  aorta?,  are  valves  which  control  the 
flow  of  the  blood,  as  will  be  described  on  a  later  page.  On  the 
dorsal  side  of  the  heart  is  a  large  thin-walled  chamber,  the  ve- 
nus  sinus  (Fig.  92  B,  vs),  into  which  open  the  veins  that  bring 
the  blood  back  from  the  body.  This  sinus  opens  into  the  right 
auricle,  which  thus  receives  all  the  blood  which  flows  back  to 
the  heart  from  all  parts  of  the  body,  except  the  lungs.  The 
blood  from  the  lungs  empties  into  the  left  auricle  by  two  small 
veins,  one  from  each  lung;  Fig.  92  pv. 
The  blood  vessels  ramify  all  over  the  body  in  a  very  complex 


189 


\ 


FIG.  92. — DIAGRAM  OF  THE  CIRCULATION  OF  THE  FROG 

The  veins  are  represented  in  shaded  lines  and  those  entering  the  heart  from 
in  front  are  shown  only  on  one  side.  A,  the  general  circulation  shown  from 
below. 


a,  abdominal  vein; 
ao,  aorta; 
6,  brachial; 
ba,  bulbus  arteriosus; 
ca,  cceliac  axis; 
co,  carotid  artery; 
CM,  cutaneous  artery; 

hv,  hepatic  vein; 
in,  intestine; 
jv,  jugular  vein; 
I,  lingual  artery; 
la,  left  auricle; 
pro,  pre  vena  cava; 
ptc,  post  vena  cava; 

pu,  pulmonary  artery; 
pv,  portal  vein; 
ra,  right  auricle; 
rp,  renal-portal  vein; 
sp,  spermary; 
v,  ventricle. 

B,  The  heart  from  below,  showing  the  venus  sinus  cut  open. 
ao,  aorta;  puv,  pulmonary  vein;         ptc,  post  vena  cava; 

la,  left  auricle;  pvc,  pre  vena  cava;  vs,  venus  sinus, 

ra,  right  auricle; 


190  BIOLOGY 

system.  The  arteries,  which  take  blood  away  from  the  heart, 
are  thick-walled  and  elastic;  while  the  veins,  which  bring  it 
back  again,  are  thin-walled.  The  distribution  of  the  chief 
blood  vessels  is  shown  in  Figure  92.  The  bulbus  arteriosus 
soon  divides  into  two  branches  that  turn  backward  and  finally 
unite  with  each  other  in  the  abdomen  beneath  the  stomach. 
These  two  branches,  the  aortae,  ao,  give  off  in  their  course 
many  vessels,  the  chief  of  which  are  the  lingual  to  the  tongue, 
I,  the  carotid  to  the  head,  co,  the  brachial  to  the  arms,  6, 
and  the  coeliac  axis  to  the  organs  of  the  abdomen,  ca.  After 
dividing  many  times  into  smaller  and  smaller  branches,  the 
arteries  finally  break  up  into  an  immense  number  of  minute 
thin-walled  vessels  called  capillaries  (Lat.  capillus  =  hair). 
These  are  microscopic,  but  of  great  importance,  since  all  of 
the  interchanges  between  the  blood  and  the  tissues  of  the  body 
take  place  through  them;  Fig.  101.  The  blood,  after  passing 
through  the  capillaries,  enters  again  into  a  series  of  vessels  of 
constantly  increasing  diameter  and  finds  its  way  back  to  the 
heart.  These  larger  returning  vessels  are  veins,  and  they 
unite  with  others,  until  finally  a  few  large  veins  are  formed 
which  empty  into  the  sinus;  Fig.  92  B.  The  blood  vessels 
thus  form  a  closed  system,  and  the  blood  that  leaves  the  heart 
returns  without  leaving  the  vessels.  The  blood  that  goes  to 
the  intestine  by  the  coeliac  axis,  ca,  passes  through  two  series 
of  capillaries  before  again  entering  the  heart.  It  first  passes 
into  capillaries  in  the  intestine,  where  it  receives  nutriment 
absorbed  from  the  food;  then  it  is  collected  into  a  large  vein, 
the  portal  vein,  pv,  which  enters  the  liver,  and  breaks  up  into 
another  system  of  capillaries;  then,  by  the  way  of  the  hepatic 
vein  (Gr.  hepar  =  liver),  hv,  it  enters  into  the  large  posterior 
vena  cava  (Figs.  A  and  B,  ptc),  which  leads  to  the  venus  sinus. 
This  system  of  veins  and  capillaries  in  the  liver  is  called  the 
portal  system.  Part  of  the  blood  that  goes  to  the  legs  also 
has  a  double  system.  It  first  enters  the  capillaries  in  the 
muscles  of  the  legs,  and  on  its  way  back  a  part  of  it  passes 


THE  FROG  191 

through  the  kidneys,  where  it  is  again  broken  up  into  capil- 
laries. This  is  the  course  of  the  blood  which  returns  from 
the  leg  through  the  renal-portal  vein  (Fig.  92  rp),  but  the  rest 
of  the  blood  from  the  legs  is  diverted  to  an  abdominal  vein,  a, 
which  enters  the  heart  without  passing  through  the  liver. 
Both  the  liver  and  the  kidneys  have  their  own  supply  of  blood 
from  the  aorta,  as  well  as  that  received  from  the  veins. 

The  vessels  thus  far  described  are  called  the  systemic  cir- 
culation, in  distinction  from  the  pulmonary  circulation,  which 
is  the  circulation  in  the  lungs.  The  lungs  are  elastic  bags 
(Fig.  92),  capable  of  much  expansion  when  inflated  with  air, 
but  collapsing  if  the  air  is  removed.  They  are  connected  with 
the  mouth  by  the  larynx,  which  opens  at  the  base  of  the  tongue 
through  the  glottis.  Through  the  glottis  and  the  larynx  air 
is  taken  into  the  lungs  to  purify  the  blood.  The  arteries  which 
supply  the  lungs,  the  pulmonary  arteries,  pu,  arise  from  the 
main  arteries  near  the  heart.  From  each  of  these  an  artery 
is  given  off  to  the  skin  under  the  arm,  the  cutaneous,  cu.  Since 
in  the  lungs  the  blood  is  purified  by  the  oxygen  of  the  air, 
and  through  the  skin  it  is  purified  by  the  oxygen  in  the  water, 
the  frog  can  live  either  in  the  water  or  in  the  air,  i.  e.,  it  is 
amphibious.  The  blood  that  is  purified  in  the  lungs  enters  the 
heart  again  by  a  pulmonary  vein,  puv,  which  flows  into  the  left 
auricle.  The  pure  blood  in  the  left  auricle  is  thus  kept  separate 
from  the  impure  blood  in  the  right  auricle,  but  as  soon  as  the 
auricles  contract  the  blood  of  both  auricles  is  forced  into  the 
single  ventricle,  and  intermingles.  Although  the  blood  in  the 
ventricle  is  really  mixed,  still  the  blood  upon  the  right  side  of  it, 
since  it  received  blood  directly  from  the  right  auricle,  will  con- 
tain more  impure  blood  than  that  on  the  left  side,  which  is 
connected  directly  with  the  left  auricle.  The  pure  and  impure 
blood  are  kept  partly  separate  by  muscular  partitions  ex- 
tending irregularly  through  the  ventricle. 

The  blood  is  composed  of  a  colorless  liquid,  called  the  plasma, 
in  which  float  two  types  of  corpuscles.  The  larger,  the  red 


192 


BIOLOGY 


corpuscles  (erythrocytes)  (Gr.  erythros  =  red  +  cytos  =  cell)  (Fig. 

93  re),  are  oval  in  shape;  their  red  color  is  due  to  the  haemo- 
globin, which  is  in  the  cor- 
instead  of  in  the 
as  in  the  case  of 


rc . 


,  .  7:L   .••.    I*  ~ 

in.  mm 


FIG.  93. — BLOOD  OF  THE  FROG,  HIGHLY 
MAGNIFIED 

lu,  leucocytes  or  white  corpuscles; 
rc,  red  corpuscles. 


puscles, 
plasma, 

the  earthworm.  The  white 
corpuscles  (leucocytes)  (Gr. 
leukos  =  white  +  cytos  = 
cell),  lu,  are  smaller  than 
the  red  corpuscles,  and  are 
able  to  force  themselves 
through  the  walls  of  the 
capillaries,  and  wander  in- 
definitely through  the  tis- 
sues. There  is  a  third  type 
of  very  minute  bodies  in  the 
plasma,  called  platelets,  of  which  little  is  known. 

Lymph  System. — Besides  the  blood  vessels,  the  frog  has  a 
system  of  much  smaller  lymph  vessels  in  the  skin,  the  intestine, 
and  other  parts  of  the  body.  These  are  thin  walled  and  filled 
with  a  colorless  liquid,  the  lymph,  and  are  so  delicate  the,t 
they  can  be  seen  only  in  specially  prepared  specimens.  In 
places  these  vessels  are  connected  with  spaces  between  the 
tissues,  lacunae,  and  with  the  large  cavities  of  the  body.  In 
the  intestine  the  lymph  vessels  receive  a  special  name,  the 
lacteals.  Lymphatic  glands  are  found  in  connection  with  the 
lymph  vessels,  and  in  the  frog  there  are  also  two  pairs  of  lymph 
hearts,  whose  contraction  propels  the  lymph  in  its  circulation. 
The  Nervous  System. — The  nervous  system  consists  of:  (1) 
The  cerebrospinal  axis,  (2)  The  cranial  nerves,  (3)  The  sympa- 
thetic system. 

Cerebrospinal  Axis. —  The  brain  and  spinal  cord  are  on 
the  dorsal  side  of  the  animal,  within  the  neural  canal  and  the 
cavity  of  the  skull;  Fig.  94.  The  brain  consists  of  several 
distinct  parts.  Beginning  in  front  they  are  as  follows:  The 


THE  FROG 


103 


olfactory  lobes,  ol,  the  cerebral  hemispheres,  ce,  the  thalamen- 
cephalon,  th,  the  optic  lobes,  op,  the  cerebellum,  cb,  and  the 
medulla  oblongata,  m.  The  cere- 
bellum is  very  small,  and  the  me- 
dulla appears  to  be  only  an  en- 
larged continuation  of  the  spinal 
cord.  In  the  latter  there  is  a 
large  triangular  cavity,  roofed 
over  by  a  thin  membrane  con- 
taining blood  vessels  (choroid 
plexus) .  The  cavity  is  called  the 
fourth  ventricle,  and  it  commu- 
nicates with  other  cavities  in  the 
brain.  On  top  of  the  thalamen- 
cephalon  is  a  small  body,  the 
pineal  gland  or  the  epiphysis,  pi. 
The  under  side  of  the  thalamen- 
cephalon  is  produced  into  a 
process  directed  backward,  the 
infundibulum,  which  ends  in 
another  small  body,  the  pituitary 
body  or  the  hypophysis. 

The  cerebrum  and  thalamen- 
cephalon  together  constitute  the 
forebrain,  the  optic  lobes  form 
the  mid-brain,  and  the  cerebel- 
lum and  medulla  form  the  hind-brain.  The  relative  devel- 
opment of  these  different  parts  varies  widely  in  different 
animals,  and  in  the  higher  vertebrates  the  cerebrum  becomes 
much  the  largest  part  of  the  brain,  this  development  reaching 
its  maximum  in  the  human  species. 

From  the  posterior  part  of  the  medulla  the  spinal  cord,  sp, 
extends  through  the  spinal  column,  tapering  to  a  minute  fila- 
ment, which  extends  a  short  distance  into  the  urostyle.  The 
brain  and  spinal  cord  are  covered  by  two  membranes,  an  outer 


FIG.  94. —  THE  CENTRAL 
NERVOUS  SYSTEM 

Shown  in  position  in  the  skull  and 
spinal  column. 

cb,  cerebellum;  op,  optic  lobe; 

ce,  cerebrum;  pi,  pineal  body; 

m,  medulla  oblon-  sp,  spinal  cord; 

gata;  th,   thalamen- 

ol,  olfactory  sac;  cephalon. 


194  BIOLOGY 

tough  one  called  the  dura  mater,  and  a  more  delicate,  inner 
membrane,  the  pia  mater. 

The  Craniospinal  Nerves. — Twenty  pairs  of  nerves  arise 
from  the  brain  and  cord,  —  ten  from  the  brain,  and  an  equal 
number  from  the  cord.  Those  from  the  brain,  the  cranial 
nerves,  supply  the  organs  of  special  sense  and  the  muscles 
and  other  organs  of  the  head,  the  heart,  lungs,  and  stomach. 
They  are  as  follows:  — 

1.  The  olfactory  nerves,  from  the  olfactory  lobes    supplying 
the  nasal  cavities. 

2.  The  optic  nerves.    These  two  nerves   arise  from  the  optic 
lobes,  cross  each  other  to  form    the    optic  chiasm,  and  then 
each  passes  to  the  eye  on  the  opposite  side  of  the  head. 

3.  Motor  ocularis,  supplying  the  muscles  of  the  eye. 

4.  Patheticus,  supplying  the  muscles  of  the  eye. 

5.  Trigeminal,  supplying  the   sides  of  the  head  (sensory). 

6.  Abducens,  supplying  the  muscles  of  the  eye. 

7.  Facial,  supplying  the  sides  of  the  head  (chiefly  motor) , 

8.  Auditory,  supplying  the  ear. 

9.  Glossopharyngeal,  supplying  the  pharynx  and  the  tongue 
(sensory). 

10.  Pneumogastric,  supplying  the  larynx,  the  heart,  and  the 
stomach. 

From  the  spinal  cord  arise  ten  pairs  of  spinal  nerves,  one 
between  the  skull  and  the  first  vertebra,  and  one  between 
each  vertebra  and  the  next;  Fig.  95.  The  first  supplies 
the  tongue  (motor);  the  second  and  third  unite  to  form  the 
nerve  going  to  the  arm,  the  brachial  nerve  (Lat.  brachium  = 
arm);  the  fourth,  fifth,  and  sixth  supply  the  middle  region 
of  the  body;  and  the  seventh,  eighth,  and  ninth  unite  by  cross 
branches  to  form  the  sciatic  plexus  (Lat.  plectare  =  to  braid), 
from  which  arise  the  nerves  that  supply  the  leg,  the  sciatic 
nerve,  which  is  the  largest  in  the  body;  the  tenth  nerve  supplies 
the  region  of  the  urostyle.  Each  nerve  arises  from  the  cord 
by  two  roots,  of  which  the  anterior  root  carries  impulses  away 


THE  FROG 


195 


from  the  brain  (efferent  fibers),  and  the  posterior  root  carries 
impulses  toward  the  brain  (afferent  fibers). 

The  Sympathetic  System. — In  the  abdominal  cavity,  lying 
on  each  side  of  the  spinal  column,  is  a  chain  of  minute  nerve 
ganglia,  ten  in  number,  which  are  also 
connected  with  the  spinal  nerves;  Fig. 
95  sy.  These  constitute  the  sympa- 
thetic system.  From  these  two  chains 
of  ganglia  minute  nerves  are  given  off, 
chiefly  to  supply  the  intestine,  the  kid- 
ney, and  the  other  organs  of  the  ab- 
domen. Although  connected  with  the 
spinal  nerve,  the  sympathetic  system  is 
quite  distinct  and  has  special  functions. 

The  microscope  shows  that  the  nerv- 
ous system,  like  that  of  the  earthworm, 
is  composed  of  an  enormous  number  of 
neurons,  each  with  its  cell  body,  dendrites, 
and  axon.  These  are  massed  in  the 
brain  and  cord,  and  there  are  many  also 
in  the  ganglia  outside  of  the  cord.  They 
are  so  situated  that  part  of  them  carry 
impulses  to  the  center,  and  part  of  them 
carry  them  in  the  reverse  direction. 
Their  numbers  are  greater  and  their  re- 
lations more  complex  than  those  of  the 
earthworm. 

The  Sense  Organs. — At  the  periph- 
eral end  of  all  of  the  sensory  nerves 
are  found  very  complicated  organs,  con- 
structed so  as  to  be  affected  by  certain 
external  stimuli.  When  they  are  stimu- 
lated impulses  start  from  them  and  pass 

over   the    afferent   nerves  to  the  brain,  where  they  become 
sensations.  They  are  the  sensory  organs  and  are  as  follows: — 


FII 


vn. 


FIG.  95.  —  DIAGRAM 
SHOWING  THE  RELA- 
TION OF  SYMPATHETIC 
SYSTEM  TO  THE 
SPINAL  NERVES 

The  sympathetic  chain  of 
one  side  only  is  shown.  The 
spinal  nerves  are  indicated  by 
Roman  numerals;  sy,  sympa- 
thetic nerve ;  syg ,  sympathetic 

ified  from  Parker.) 


196 


BIOLOGY 


Olfactory  organs. — Just  within  the  nostrils  are  two  little 
cavities  occupied  by  the  olfactory  sacs.  In  these  sacs  the 
olfactory  nerves  are  distributed,  ending  in  delicate  nerve  cells, 
which  are  sensitive  to  odors;  Fig.  96  A. 

Cornea 


umor\Iris 


.1 


FIG.  96. — TERMINATION  OF 
SENSORY  NERVE  CELLS 

A,  Olfactory  cells;  B,  cells  in 
the  retina,  sensitive  to  the  light, 
showing  the  rods  on  the  left  and 
cones  on  the  right. 

(Dogiel  and  Gaupp.) 


-071 

FIG.  97. — DIAGRAMMATIC  CROSS  SECTION  OP 
THE  EYE  OF  THE  FROG 


ch,  choroid  coat; 

I,  the  suspensory  ligament; 

on,  optic  nerve;         (Retzius.) 


,  retina; 

:,  sclerotic  coat. 


The  eyes. — The  eyes  are  large,  spherical  organs,  planned 
after  the  structure  of  the  vertebrate  eyes  in  general.  Figure  97 
is  a  cross  section  of  an  eye  showing  the  important  parts.  It 
is  a  spherical  chamber,  the  walls  of  which  are  opaque,  except 
in  front,  where  they  are  transparent,  and  act  like  the  dark 
chamber  of  a  camera.  The  walls  of  the  chamber  are  made  of 
several  layers.  In  the  very  front  is  the  cornea,  presenting  a 
transparent  curved  surface.  The  back  part,  comprising  about 
two-thirds  of  the  chamber  wall,  is  made  of  three  layers.  On 
the  outside  is  a  sclerotic  coat,  sc,  composed  of  fibrous  tissue 
and  cartilage;  next  to  this  a  thin  coat  containing  pigment, 
the  choroid,  ch,  and  inside  of  this  a  still  thinner  retina,  r,  which 


THE  FROG  197 

is  the  sensitive  part  of  the  eye.  At  the  back  of  the  chamber 
is  an  opening  through  which  the  optic  nerve  enters,  on.  After 
entering  the  eye  the  nerve  spreads  out  on  the  retina,  where  it 
is  affected  by  the  light  entering  the  eye.  The  chamber  of  the 
eye  is  divided  into  two  parts  by  a  large  spherical,  transparent, 
crystalline  lens,  held  in  position  by  several  bands  of  fibers, 
shown  at  I.  Anteriorly  the  lens  is  partly  covered  by  an  opaque 
membrane,  really  a  continuation  of  the  choroid,  which  grows  out 
from  the  wall  of  the  chamber  on  all  sides.  This  is  the  iris,  and 
it  covers  the  outer  part  of  the  lens,  except  in  the  middle,  where 
the  lens  is  not  covered.  This  opening  is  the  pupil,  and  serves 
to  allow  light  to  enter.  The  iris  contains  pigment  cells,  which 
give  the  eye  its  color.  Each  of  the  two  chambers  of  the  eye 
is  filled  with  a  transparent  fluid.  That  lying  between  the 
cornea  and  the  lens  is  the  aqueous  humor,  and  that  back  of  the 
lens,  which  is  rather  more  solid,  is  the  vitreous  humor.  The 
retina,  which  lines  the  eye  chamber,  is  an  extremely  complicated 
organ,  made  of  hundreds  of  thousands  of  end  organs  of  sensi- 
tive nerves.  It  is  a  complex  of  neuron  bodies,  dendrites,  and 
axons  (Fig.  96  B)t  and  is  highly  sensitive  to  the  light,  which 
is  focused  upon  it  by  the  lens.  Attached  to  the  ball  of  the  eye 
are  six  muscles,  by  means  of  which  it  can  be  rotated  in  any 
direction: 

The  ears. — The  frog  has  no  external  ears.  Just  back  of  the 
eyes  are  two  rounded,  flat  depressions,  each  formed  by  a  mem- 
brane which  covers  the  real  ear.  If  the  thin  skin  which  covers 
this  area  be  removed,  a  rather  tough,  flat  membrane  will  be 
found  beneath,  which  is  the  tympanic  membrane  proper.  This 
membrane  extends  over  a  shallow  conical  cavity,  called  the 
tympanum  or  ear-drum.  This  cavity  connects  below  with 
the  mouth  through  the  eustachian  tube.  Extending  across 
this  is  a  slender  bar  of  bone  and  cartilage,  called  the  columella. 
This  is  attached  to  the  membrane  at  one  end  and  connected 
with  the  inner  ear  at  the  other,  and  transmits  vibrations  of 
the  membrane  to  the  inner  ear,  the  real  organ  of  hearing. 


198 


BIOLOGY 


FIG. 


an 


\. THE    INTERNAL  EAR 

OF   THE    FROG 


an,  auditory  nerve; 
c,  semicircular  canals; 

(Retzius.) 


s,  saccule; 
u,  utricle. 


The  inner  ear,  which  is  :he  true  sensory  end  organ  of  the  audi- 
tory nerve,  lies  embedded  in  the  bones  of  the  skull.  Its  general 
appearance  may  be  seen  from  Figure  98.  It  is  quite  a  compli- 
cated organ,  and  the  auditory  nerve  enters  in  and  finally 

terminates  in  delicate  endings, 
which  are  readily  stimulated  by 
the  vibrations  brought  from  the 
exterior  through  the  membrane 
and  the  columella.  The  canals 
shown  at  c,  the  semicircular  ca- 
nals, have  a  function  related  to 
balancing  the  body  and  keeping 
it  in  an  upright  position,  i.  e., 
equilibrium. 

Other  senses. — The  sense  of  smell 
is  located  in  the  nostrils.  These 
openings  lead  into  little  olfactory 

sacs  just  within  the  bones,  and  the  air  which  enters  them 
passes  through  the  bones  into  the  mouth  by  openings  on  the 
roof  of  the  mouth  called 
internal  nares.  The  ol- 
factory nerve  is  expanded 
in  the  olfactory  sac, 
where  vapors  that  may 
be  in  the  air  affect  it. 

The  sense  of  taste  is 
situated  on  the  tongue, 
within  which  are  end  or- 
gans sensitive  to  liquids. 
Only  substances  dis- 
solved in  liquids  are  ca- 
pable of  affecting  these 
end  organs;  Fig.  99. 

The  touch  and  pressure  senses  are  located  in  the  skin.    Scat- 
tered over  the  body  generally  are  numerous  end  organs,  which 


n 


FIG.  99. — SECTION  OF  TONGUE  OF  FROG 

n,  nerve  ending;    nc,  nerve  cells;     ne,  nerve  trunk. 
(Gaupp.) 


THE  FROG  199 

form  the  termination  of  the  sensory  nerves.  They  are  of  different 
kinds,  and  doubtless  have  different  functions,  but  all  are  associ- 
ated with  what  is  in  general  called  the  sense  of  feeling  or  touch. 

The  Excretory  Organs. — Lying  in  the  back  part  of  the  ab- 
domen near  the  legs  are  two  flat,  rather  oval  bodies,  one  on 
either  side  of  the  middle  line,  the  kidneys;  Fig.  92.  Each  is 
abundantly  supplied  with  blood  vessels,  a  fact  which  indicates 
important  functions.  Microscopic  study  shows  them  to  be  made 
of  many  coiled  tubes,  similar  to  the  nephridia  of  the  earth- 
worm. These  tubes  remove  excreted  products  from  the  blood 
which  passes  through  them.  From  the  outer  side  of  each  a 
small  duct,  the  ureter,  passes  backward  toward  the  cloaca, 
where  it  empties  into  the  bladder  (Fig.  90  bl) ,  a  rather  large  two- 
lobed  sac,  which  may  be  filled  with  the  urine  secreted  by  the 
kidney,  or  may  collapse  when  empty.  It  opens  into  the  cloacal 
chamber,  and  its  contents  are  discharged  through  the  common 
cloacal  opening.  (In  man  a  special  duct,  the  urethra,  leads 
from  the  bladder  to  the  exterior.) 

Reproductive  Organs. — The  two  sexes  in  the  frog  are  in 
separate  individuals,  thus  differing  from  the  condition  found 
in  the  earthworm.  The  male  may  be  distinguished  externally 
by  a  thick  pad  on  the  under  side  of  its  thumb,  which  is  rather 
large  in  the  spring,  but  small  at  other  seasons  of  the  year. 
The  spermaries  are  found  in  the  male  at  the  upper  end  of  the 
kidneys;  Fig.  92  sp.  They  are  two  in  number,  rounded  or 
oval  in  shape,  and  of  a  light  yellowish  color.  Attached  to  them 
are  usually  several  branching  masses  of  yellow  fat.  The  sperm 
produced  in  the  spermaries  are  carried  through  some  delicate 
ducts  into  the  kidney.  These  ducts,  the  vasa  efferentia,  pass 
through  the  kidneys  and  empty  into  the  ureters,  which  lie 
on  their  outer  edge.  The  ureters  in  the  frog  thus  serve  for  the 
exit  of  both  the  kidney  secretion  and  the  secretions  from  the 
spermaries.  These  ureters  are,  in  some  species  of  frogs,  en- 
larged into  a  small  sac  just  at  the  point  where  they  enter  the 
cloacal  chamber,  and  in  these  sacs  the  sperms  are  stored  until 


200 


BIOLOGY 


the  frog  is  ready  to  discharge  them  at  the  time  of  copulation. 
These  sacs  are  called  seminal  vesicles.  Some  species  of  frogs 
do  not  have  such  vesicles. 

In  the  females  the  ovaries  are  situated  in  a  position  cor- 
responding to  that  of  the  spermaries  in  the  male;  Fig.  100  ov. 

During  the  late  spring  and 
summer  they  are  rather  small, 
slightly  folded,  leaf-like  or- 
gans, not  much  larger  than 
the  spermaries,  though  differ- 
ing in  shape.  In  the  fall  of 
the  year  the  eggs  in  these 
ovaries  begin  to  grow,  causing 
the  ovaries  to  become  greatly 
expanded.  During  the  fall  the 
ovaries  are  usually  greatly 
swollen  and  completely  fill 
the  abdominal  cavity,  almost 
concealing  the  other  organs. 
The  oviducts  that  carry  the 
eggs  to  the  exterior  are  not 
directly  connected  with  the 
ovaries.  They  are  very  much 
coiled  tubes,  o,  lying  beside  the 
kidneys,  each  ending  at  its  an- 
terior end  in  a  funnel-shaped 
opening.  From  this  opening 
the  tube  passes  backward 
beside  the  kidneys,  and,  after 
making  many  coils,  finally 
opens  into  the  cloacal  chamber  at  the  back.  Just  before  its 
termination  it  is  swollen  into  a  rather  large,  thin-walled  cham- 
ber, the  uterus,  ut,  in  which  the  eggs  may  be  stored  for  a  time 
after  passing  through  the  oviducts  before  the  final  egg  laying. 
These  long  ducts  vary  greatly  in  size  at  different  seasons, 


FIG.  100. —  REPRODUCTIVE  ORGANS 
OF  A  FEMALE  FROG,  ATTACHED  TO 
THE  KIDNEYS 


ki,  kidneys; 

o,  oviduct; 

ov,  ovary  filled  with  eggs; 


ut,  uterus; 
c,  cloacal 
chamber. 


THE  FROG  201 

being  small  in  the  summer,  but  enlarging  with  the  enlargement 
of  the  ovaries,  and  swelling  greatly  in  the  early  spring  pre- 
paratory to  egg  laying.  In  the  walls  of  the  oviducts  are  numer- 
ous little  glands,  whose  function  is  to  secrete  material  around 
the  egg  to  form  the  shell  or  other  protective  covering.  They 
are  nidamental  glands  (Lat.  nidus  =  a  nest). 

It  will  be  seen  that  the  sexual  organs  and  the  kidneys  are 
very  closely  connected.  They  lie  close  together,  have  a  com- 
mon opening,  and  in  the  male  the  same  duct,  the  ureter,  serves 
for  the  exit  of  the  sperms  and  the  urine.  A  similar  close  rela- 
tion is  found  in  other  vertebrates,  and  a  study  of  the  develop- 
ment of  the  animals  shows  that  their  ducts  are  originally 
derived  from  the  same  organ  in  the  embryo.  The  two  systems 
together  are  known  as  the  urogenital  system.  In  the  frog 
this  system  opens  to  the  exterior  with  the  intestine  by  the 
single  common  cloacal  opening.  In  higher  animals  they  may 
have  separate  openings. 

LABORATORY  WORK  UPON  THE  FROG 

For  a  detailed  dissection  of  the  frog,  reference  must  be  made  to  some 
of  the  numerous  laboratory  manuals.  The  brief  general  directions  given 
below  will  be  sufficient  to  illustrate  the  topics  discussed  in  the  text,  and 
at  least  this  amount  of  laboratory  work  is  necessary  to  make  the  text 
properly  intelligible. 

If  the  specimens  are  obtained  alive  they  should  first  be  killed  with 
chloroform,  and,  while  still  fresh,  all  of  the  points  in  the  external  anatomy 
should  be  made  out.  Note  should  be  made  of  the  following:  head;  body; 
.absence  of  tail;  the  loose  skin,  attached,  however,  at  certain  points;  arms; 
numbers  of  fingers;  legs;  number  of  toes;  web  between  the  toes;  mouth; 
nostrils;  eyes  with  eyelids;  ears;  cloacal  opening.  Open  the  mouth  and 
note  tongue;  glottis;  gullet. 

The  dissection  of  the  organs  of  the  abdomen  can  best  be  made  with  a 
freshly  killed  specimen,  but  it  may  be  done  satisfactorily  with  animals 
preserved  in  alcohol  or  formalin.  The  dissection  of  the  brain  and  spinal 
cord  should  always  be  made  upon  animals  preserved  in  alcohol,  since 
these  organs  are  too  soft  to  handle  in  fresh  specimens.  A  mounted  skele- 
ton of  the  animal  should  be  at  hand  for  study  and  comparison  with  the 
animal  under  dissection. 


202  BIOLOGY 

The  order  of  dissection  given  below  is  so  planned  as  to  make  it  pos- 
sible to  do  practically  all  of  tlie  dissection  upon  a  single  specimen.  The 
specimen  may  be  preserved  in  formalin  and  the  work  carried  out  at  leisure. 
If  the  order  given  is  followed,  it  is  possible  to  have  a  large  class  working 
at  the  same  time,  and,  when  the  work  is  finished,  all  of  the  important 
parts  in  the  anatomy  will  have  been  made  out,  except  the  skull  and  the 
shoulder  girdle,  these  having  of  necessity  been  destroyed  in  opening  the 
body  and  in  exposing  the  brain.  If  frequent  references  are  made  to  the  de- 
scription of  the  frog  given  in  the  text,  the  brief  description  here  given 
will  be  sufficient  to  make  a  satisfactory  dissection. 

Open  the  frog  by  a  median  ventral  incision,  made  with  scissors,  ex- 
tending from  the  legs  forward  to  the  sternum,  cutting  through  both  skin 
and  flesh.  The  blunt  end  of  the  scissors  is  then  to  be  thrust  under  the 
sternum,  and  this  girdle  of  bones  is  to  be  cut  through.  This  will  make 
it  possible  to  open  the  abdomen,  pinning  out  the  flaps  of  the  abdominal 
walls  and  the  arms  so  as  to  expose  the  organs  of  the  abdomen.  If  the  frog 
is  a  freshly  killed  specimen,  all  of  the  subsequent  study  of  the  viscera 
should  be  made  with  the  animal  immersed  in  water.  If  the  frog  is  a  pre- 
served specimen,  this  is  not  so  necessary. 

The  organs  of  the  abdomen  may  now  be  studied.  The  following  parts 
should  be  made  out  without  any  further  dissection,  being  disclosed  simply 
by  pushing  the  organs  one  after  the  other  to  one  side,  and  they  may  bo 
examined  conveniently  in  the  following  order:  liver;  heart;  large  arteries 
around  the  heart;  veins  entering  the  heart;  stomach;  intestine;  gall  blad- 
der; rectum;  mesentery,  which  contains  blood  vessels  that  may  be 
traced  to  the  liver. 

In  opening  the  body,  if  the  specimen  is  a  fresh  one,  there  is  danger 
that  some  of  the  blood  vessels  may  be  cut,  making  it  difficult  or  impos- 
sible to  follow  the  blood  vessels.  In  order  to  work  out  the  blood  vessels 
satisfactorily,  it  is  necessary  to  have  an  injected  specimen.  These  may 
be  bought  of  dealers  in  natural  history  supplies,  or  the  injection  may  be 
done  by  the  instructor. 

If  the  specimen  is  a  female,  the  body  cavity  will,  at  certain  seasons  of 
the  year,  be  filled  with  an  enormously  expanded  ovary,  filled  with  eggs. 
In  order  to  make  out  the  other  abdominal  organs,  these  must  be  removed 
carefully,  so  as  not  to  injure  the  other  parts.  After  they  have  been 
removed  there  will  appear  lying  on  either  side  of  the  back  part  of  the 
abdomen  the  very  much  enlarged  oviduct,  showing  as  a  much  coiled 
tube.  This  should  also  be  removed,  note  .being  made  of  its  connection 
with  the  cloacal  chamber  behind.  If  the  ovary  is  not  thus  enlarged,  or 
if  the  specimen  is  a  male,  it  is  not  necessary  to  remove  the  reproductive 
organs  to  show  the  other  features. 


THE  FROG  203 

With  the  organs  all  in  position,  now  make  out  the  rectum;  the  bladder; 
the  spleen;  the  cloacal  chamber;  the  kidneys;  the  spermaries;  the  ovaries 
and  oviduct  in  the  female,  and  the  spermaries  and  vas  deferens  in  the  male. 

Remove  the  heart,  liver,  stomach,  and  intestines.  This  will  disclose 
the  lungs;  the  two  systemic  arteries  uniting  to  form  the  dorsal  aorta, 
which  should  be  traced  to  where  it  divides  to  supply  the  legs;  the  nerves 
to  the  arms;  nerves  to  the  back;  three  large  nerves  arising  from  the  back- 
bone and  extending  toward  the  legs,  and  finally  uniting  to  form  the  sciatic 
plexus,  from  which  arise  the  large  nerves  entering  the  leg.  By  lifting  up 
the  aorta  gently,  delicate  branches  of  the  sympathetic  system  may  be 
seen  and  traced  to  their  ganglia. 

One  leg  of  the  animal  should  be  dissected  to  make  out  the  muscles, 
nerves,  bones,  and  joints.  The  muscles  should  be  separated  from  each 
other  and  traced  to  their  origin  and  insertion,  special  notice  being  taken 
of  the  long  tendons  extending  from  the  lower  muscles  down  to  the  toes. 
In  the  joints,  note  the  freedom  of  motion  of  the  bones;  the  tendons,  which 
extend  over  them;  the  rather  loose  ligaments  that  unite  the  bones;  the 
readiness  with  which  the  bones  come  apart  when  the  ligaments  are  cut; 
the  smooth  surfaces  of  the  ends  of  the  bones;  and  the  cartilage  that  covers 
their  ends.  (If  there  is  time  for  more  careful  dissection,  reference  must  be 
made  to  laboratory  guides  on  the  dissection  of  the  frog.)  Clean  all  of  the 
soft  parts  from  the  bones  of  the  leg,  separating  and  identifying  each  bone. 

Examine  the  eyelids;  the  iris;  the  pupil.  Make  an  incision  through 
the  iris  and  remove  the  lens;  note  the  cavity  of  the  eye  behind  the  lens. 
Cut  an  incision  through  the  tympanic  membrane,  noting  the  shallow 
cavity  beneath  it,  the  tympanic  cavity,  the  bony  columella  extending 
across  it  to  the  skull.  A  bristle  thrust  into  the  bottom  of  this  cavity  will 
enter  the  mouth  through  the  eustachian  tube. 

Remove  with  a  knife  a  bit  of  the  -flat  bone  on  top  of  the  skull,  exposing 
the  brain;  and  then,  with  forceps  and  scissors,  break  away  the  bone  so 
as  to  expose  completely  the  brain  and  spinal  cord  down  the  back  to  the 
urostyle,  taking  care  not  to  injure  the  soft  parts.  Identify  all  parts  of 
the  brain  as  described  on  page  193. 

The  skeleton  should  be  studied  from  another  specimen.  Remove  all 
the  soft  parts  from  the  skeleton,  separating  all  the  bones.  Clean  and  iden- 
tify each  (Fig.  88),  and  compare  with  the  mounted  skeleton. 

BOOKS  FOR  REFERENCE 

ECKER,  The  Anatomy  of  the  Frog,  The  Macmillan  Co.,  New  York. 
HOLMES,  Biology  of  the  Frog,  The  Macmillan  Co.,  New  York. 
MARSHALL,  The  Frog,  The  Macmillan  Co.,  New  York. 
MORGAN,  Development  of  Frogs'  Eggs,  The  Macmillan  Co.,  New  York. 
GUYER,  Animal  Micrology,  University  of  Chicago  Press,  Chicago. 


CHAPTER  X 
THE  PHYSIOLOGY  OF  AN  ANIMAL 

HAVING  now  studied  the  structure  of  multicellular  animals, 
we  will  consider  briefly  the  functions  of  the  various  organs,  with 
special  reference  to  the  frog. 

Alimentary  System. — The  primary  purpose  of  the  alimentary 
canal  is  the  digestion  and  absorption  of  food.  The  food  of 
animals  is  always  organic,  since  animals  are  unable  to  utilize 
mineral  substances  upon  which  plants  subsist.  Animals  feed 
upon  the  substances  manufactured  by  plants:  starch,  the  first 
product  of  photosynthesis,  may  serve  animals  for  food,  and  the 
same  is  true  of  sugar,  fats,  and  proteids.  These  foods  are  usually 
in  a  solid  form  when  taken  into  the  animal's  mouth,  and  in  order 
to  be  of  any  use  they  must  pass  from  the  alimentary  canal  into 
the  blood  vessels.  Solid  food  is  incapable  of  passing  through  the 
intestinal  walls,  and  must  be  changed  so  that  it  can  be  dissolved 
in  the  liquids  of  the  alimentary  canal,  a  process  called  digestion. 
Digestion  is  brought  about  by  digestive  fluids  which  are  secreted 
by  digestive  glands  within  the  alimentary  tract.  The  frog  has 
no  salivary  glands  such  as  man  possesses,  and  the  first  digestive 
glands  are  in  the  walls  of  the  stomach.  These  are  microscopic 
in  size  and  are  called  gastric  glands.  They  are  present  in  large 
numbers  and  secrete  the  gastric  juice,  which  is  poured  directly 
upon  the  food  after  it  reaches  the  stomach.  A  second  digestive 
gland  is  the  pancreas,  lying  just  below  the  stomach  and  pouring 
its  secretion,  the  pancreatic  juice,  by  a  special  duct  into  the 
intestine,  close  to  the  stomach.  This  is  mixed  with  the  food 
just  as  it  leaves  the  stomach  and  after  it  has  been  acted  upon  by 
the  gastric  juice.  By  the  action  of  these  two  digestive  fluids  the 
solid  foods  are  changed  in  their  nature  and  rendered  partly 
soluble.  They  are  then  dissolved  in  the  intestinal  liquids,  becom- 
ing a  thick,  rather  slimy  mass  of  dissolved  material.  The 
different  foods  eaten  by  the  animal  are  subject  to  different 

204 


THE  PHYSIOLOGY  OF  AN  ANIMAL  205 

changes  under  the  influence  of  the  separate  digestive  fluids, 
those  secreted  by  the  stomach  producing  a  different  kind  of 
digestion  from  those  of  the  pancreas;  but  all  aid  in  rendering 
the  food  soluble. 

Absorption. — The  food  is  driven  through  the  alimentary  canal 
by  the  muscular  contractions  of  its  walls.  These  muscles  are  in 
two  sets,  one  extending  lengthwise  and  the  other  running  around 
the  intestine  in  a  circular  direction.  By  their  contraction  waves 
of  constriction  pass  along  the  intestine,  forcing  the  food  slowly 
along.  This  peculiar  writhing  motion  of  the  intestine  is  spoken 
of  as  peristalsis  (Gr.  peri  =  around  +  stalsis  =  a  compression). 

As  the  food  is  pushed  through  the  intestine  its  digestion  and 
solution  is  completed  and  it  begins  to  pass  through  the  walls 
of  the  intestine  into  the  surrounding  blood  vessels.  As  the 
intestinal  contents  pass  onward  more  and  more  of  the  nutriment 
contained  in  the  food  is  absorbed  from  it  and  enters  the  blood. 
The  undigested  and  useless  portions  of  the  food  pass  on  and  even- 
tually, in  the  form  of  faeces,  are  voided  through  the  cloacal 
opening. 

Circulation. — The  food  absorbed  into  the  blood  is  now  carried 
over  the  body  in  the  blood.  The  liquid  part  of  the  blood,  the 
plasma,  is  the  circulating  medium,  the  red  and  white  corpuscles 
having  special  functions.  The  red  corpuscles  (erythrocytes) , 
which  are  by  far  the  most  numerous,  give  the  blood  its  red  color 
and  are  associated  with  respiration.  The  white  corpuscles  (leu- 
cocytes) ,  of  which  there  are  several  kinds,  have  various  functions, 
one  of  which  is  the  removing  of  foreign  bodies  from  the  body 
and  protecting  it  from  the  attacks  of  microscopic  germs,  or  other 
irritating  substances  that  may  enter  the  tissues.  The  white 
corpuscles  with  this  power  are  called  phagocytes  (Gr.  phagein  = 
to  eat  +  cytos) ;  they  are  able  to  leave  the  blood  vessels,  by 
forcing  their  way  through  the  walls  of  the  capillaries;  Fig. 
101  leu.  They  then  migrate  among  the  tissues  and  collect  at 
any  part  of  the  body  to  guard  it  from  an  attack. 

The  blood  is  kept  circulating  through  the  vessels  by  means  of^ 


206 


BIOLOGY 


FlG.    101. —  A    SINGLE    CAPILLARY 

Showing  the  corpuscles  being   forced  through  its 
walls. 

re,  red  corpuscles; 
leu,  leucocytes; 

leu,  a  leucocyte  that  is  forcing  its  way  through  the 
walls  of  the  capillary  into  the  surrounding  tissues. 


the  heart,  which  acts  as  a  pump.  In  the  frog's  heart  there  are 
three  chambers  and  the  circulation  is  as  follows:  The  blood 
which  enters  the  heart  from  the  body,  which  is  impure  blood, 

is  received  first  into  the 
venus  sinus  (Fig.  92  B, 
vs),  and  from  here  it  en- 
ters the  right  auricle; 
Fig.  102  ra.  At  the 
same  time  pure  blood 
enters  from  the  lungs 
and  skin,  and  is  received 
in  the  left  auricle.  Now 
the  two  auricles  con- 
tract and  force  the 
blood  into  the  single 
ventricle  v,  through  the 
openings  indicated  by 
the  arrows  in  Figure 

102.  The  ventricle  thus  receives  both  pure  and  impure  blood, 
the  pure  blood  being  poured  into  its  left  side  and  the  impure 
blood  into  its  right  side.  These  two  kinds  of  blood  are  partly 
mixed,  excepft  for  a  fraction  of  a  second,  when  they  are  sep- 
arate from  each  other.  They  are  kept  from  mixing  too  quickly 
by  several  muscular  bands  stretching  from  the  walls  of  the 
heart.  But  almost  at  the  same  instant  that  the  ventricle  is 
filled  it  contracts,  and  its  contained  blood  is  forced  into  the 
large  artery,  the  bulbus  arteriosus.  This  artery,  as  will  be  seen 
from  Figure  102  ba,  opens  on  the  right  side  of  the  ventricle  and 
consequently  will  receive  first  the  blood  which  entered  the  ven- 
tricle from  the  right  auricle,  which  is  impure  blood.  Thus  impure 
blood  passes  first  into  the  arteries,  to  be  followed  by  mixed  blood 
and  finally  by  the  purer  blood  that  comes  from  the.  left  side  of 
the  ventricle,  and  hence  from  the  left  auricle.  With  each  con- 
traction of  the  heart  there  enters  the  arteries  first  a  little  impure 
blood,  then  a  little  mixed  blood,  and  finally  a  little  pure  blood. 


THE  PHYSIOLOGY  OF  AN  ANIMAL 


207 


ao- 


ba- 


From  Figure  102  pu,  it  will  be  seen  that  the  first  branch  of 
the  artery  passes  to  the  lungs.  In  the  bulbus  arteriosus  are 
valves  so  arranged  that  the  first  blood  passing  from  the  heart 
with  each  beat  goes  to  the  lungs;  after  these  are  partly  filled 
the  next  blood  passes  through  the  blood  vessels  shown  at 
ao,  down  to  the  arms  and  to  the  lower  parts  of  the  body;  and 
finally  the  last  of  the  blood  that  comes  out  with  each  beat  of 
the  heart  passes  up  into 
the  head  through  the  ar- 
tery, co.  Thus  the  most 
impure  blood  passes  to 
the  lungs,  where  it  is  pu- 
rified, the  mixed  blood 
goes  to  the  lower  parts 
of  the  body,  and  the 
purest  blood  goes  to  the 
head  and  brain.  The 
separation  of  pure  and 
impure  blood  in  the  frog 
is  not  complete,  but  the 
arrangement  just  de- 
scribed  is  such  as  to  send 
the  most  impure  blood 
to  the  organs  which  pu- 
rify it,  and  the  purest 
blood  to  the  brain  where 
the  purest  blood  is 
needed.  The  two  auricles  are  separated  from  the  ventricle  by 
valves,  va,  opening  mechanically  in  one  direction,  in  such  a  way 
that  when  the  heart  beats  the  blood  is  forced  onward  and 
never  backward. 

The  blood  passes  out  through  the  arteries  and  is  carried  by  the 
numerous  branches  into  the  various  parts  of  the  body,  the  small 
branches  breaking  up  finally  into  minute  twigs  called  capillaries, 
that  are  distributed  in  great  abundance  in  every  active  organ. 


FlG.    102. —  A   DIAGRAM   OF  THE  HEART  OF 
THE    FROG   TO   EXPLAIN  ITS  CIRCULATION 

no,  aorta,  passing  to  the  posterior  part  of  the  body; 

ba,  bulbus  arteriosus; 

co,  carotid  artery  going  to  the  head; 

la,  left  auricle; 

pu,  pulmonary  artery  passing  to  the  lung; 

ra,  right  auricle; 

v,  ventricle; 

va,   valves  separating  auricles  and  ventricle.     The 

arrows  show  the  passage  from  the  auricles  to  the 

ventricle. 

(Modified  from  Parker  and  Haswell.) 


208  BIOLOGY 

While  passing  through  these  capillaries,  the  food  materials, 
absorbed  by  the  blood  from  the  alimentary  canal,  and  the  gas 
absorbed  from  the  lungs,  pass  from  the  blood  into  the  tissues 
where  they  are  needed.  In  this  way  the  food  and  oxygen  are 
supplied  to  the  active  tissues  of  the  body.  At  the  same  time 
waste  products,  which  have  been  produced  in  the  active  tissues, 
are  returned  to  the  blood,  so  that  the  blood,  after  passing  through 
the  capillaries,  goes  back  to  the  heart  as  impure  blood.  After 
reaching  the  heart  the  impure  blood  goes  to  the  lungs,  where 
part  of  its  impurities  are  passed  off  into  the  air. 

Lymph  System. — A  part  of  the  circulatory  system  is  called  the 
lymph  system.  As  the  blood  is  flowing  in  the  capillaries  some 
of  the  liquid  plasma  soaks  through  the  walls  of  the  capillaries 
out  into  the  tissues.  When  it  reaches  the  tissues  it  is  no  longer 
called  blood  but  lymph,  and  is  a  colorless  clear  liquid  which 
bathes  every  living  cell.  This  lymph  contains,  dissolved  in  it, 
the  nutriment  absorbed  from  the  intestine;  and,  since  it  now 
actually  bathes  the  living  cells,  these  can  take  from  it  directly 
the  nourishment  they  need  for  their  activities.  Into  this  lymph 
the  living  cells  also  excrete  all  the  waste  products  that  have 
resulted  from  their  life  processes,  the  lymph  receiving  all  the 
wastes  of  the  body.  The  gases,  which  comprise  part  of  this 
waste,  pass  at  once  into  the  blood  by  diffusion;  but  the  other 
materials  remain  dissolved  in  the  lymph  and  finally  reach  the 
blood  by  the  following  course:  The  lymph  gradually  collects 
in  tiny  spaces,  lacunae,  scattered  over  the  body,  and  from  these 
flows  into  little  vessels  connecting  with  each  other,  called  lymph 
vessels.  These  small  vessels  unite  together  to  form  larger  ones 
and  the  larger  vessels  finally  empty  into  the  veins.  The  vessels 
around  the  front  end  of  the  body  converge  to  two  minute  sacs 
lying  deeply  imbedded  near  the  third  vertebra;  and  the  vessels 
in  the  hind  part  of  the  body  converge  into  similar  sacs  situated 
over  the  hips,  near  the  lower  end  of  the  urostyle.  These  four 
sacs  have  muscular  walls  and  pulsate,  and  are  called  lymph 
hearts.  When  they  beat  they  force  the  lymph  into  the  veins 


THE  PHYSIOLOGY  OF  AN  ANIMAL  209 

which  lie  near  them  and  with  which  they  are  connected.  In  this 
way  the  lymph,  which  originally  came  from  the  blood  plasma 
by  dialyzing  through  the  walls  of  the  capillaries,  returns  into  the 
blood;  thus  all  the  secreted  products  from  the  living  cells  pass 
into  the  blood,  either  directly  as  in  the  case  of  gases,  or  indirectly 
by  passing  first  into  the  lymph  and  then  emptying  with  the 
lymph  into  the  blood  vessels. 

NOTE. — A  similar  lymphatic  system  is  found  in  all  higher  animals,  but 
its  course  is  different  from  that  in  the  frog.  In  man,  for  example,  the 
lymph  rises  by  diffusion  through  the  capillaries,  and  collects  in  lacunae 
and  lymph  vessels  in  a  similar  manner.  But  there  are  no  lymph  hearts. 
The  lymph  vessels  unite  to  form  quite  large  vessels,  and  all  eventually 
empty  into  the  large  veins  in  the  neck.  There  are  two  chief  trunks  of  these 
vessels,  one  bringing  the  lymph  from  the  upper  parts  of  the  body  and 
emptying  into  the  right  jugular  vein,  and  the  other,  a  much  larger  one, 
bringing  the  lymph  from  the  lower  parts  of  the  body  and  from  the  alimen- 
tary canal  and  flowing  up  through  the  thorax,  to  empty  finally  in  the  left 
jugular  vein.  This  latter  lymph  vessel  is  called  the  thoracic  duct. 

Respiration. — The  impure  blood  from  the  heart  passes  through 
the  pulmonary  artery  to  the  lungs  (Fig.  92),  a  part  of  it  going 
into  a  small  branch,  the  cutaneous,  cu,  which  carries  it  to  the 
skin.  The  lungs  are  air  sacs  connected  with  the  mouth.  Just 
back  of  the  tongue  we  have  already  noticed  the  glottis,  which 
is  a  slit  leading  into  a  small  cavity  holding  the  vocal  cords, 
whose  vibrations  cause  the  various  sounds  produced  by  the 
animal.  This  cavity  is  the  larynx  and  it  lies  just  under  the 
throat.  At  its  inner  end  it  opens  at  once  into  the  lungs,  since 
the  frog  has  no  windpipe  (trachea)  such  as  is  found  in  animals 
with  long  necks,  like  man.  The  air  enters  the  lungs  through  the 
larynx  and,  filling  them,  comes  in  close  contact  with  the  blood, 
which  is  distributed  in  finely  divided  capillaries  in  their  walls. 
The  blood  that  goes  to  the  skin  through  the  cutaneous  artery 
is  distributed  in  fine  capillaries  and  brought  into  close  con- 
tact with  the  oxygen  which  is  dissolved  in  the  water  in  which 
the  animal  lives. 

The  haemoglobin,  which  gives  the  red  color  to  the  red  cor- 


210  BIOLOGY 

puscles,  absorbs  large  quantities  of  oxygen  as  the  blood  is  flow- 
ing through  the  lungs  and  skin.  The  oxygenated  blood  then 
passes  from  the  lungs  back  to  the  heart  and  is  pumped  out 
through  arteries  to  the  tissues.  Here  the  red  blood  corpuscles 
give  up  their  oxygen,  and  at  the  same  time  the  blood  absorbs 
carbon  dioxid  (CO2)  from  the  tissues.  When  the  blood,  there- 
fore, leaves  the  capillaries  on  its  journey  back  to  the  heart,  it 
has  left  behind  its  oxygen  and  taken  in  its  place  carbon  dioxid, 
which  it  gives  up  when  it  next  reaches  the  lungs  or  the  skin, 
at  the  same  time  taking  up  oxygen.  The  process  of  respiration 
is  therefore  a  system  of  gas  exchange. 

Metabolism. — In  the  living  tissues  the  food  and  oxygen  are 
chemically  combined,  an  oxidation  of  the  food  taking  place. 
The  chemical  changes  that  occur  are  numerous  and  result  in 
the  formation  of  new  materials  for  the  body,  producing  growth , 
development  of  muscular  activity,  and  all  of  the  other  phenom- 
ena of  life,  and  finally  resulting  in  the  appearance  of  waste 
products.  The  waste  products  are  chiefly  three :  (1)  a  gas,  carbon 
dioxid  (CO2) ;  (2)  a  liquid,  water  (H20) ;  (3)  a  solid,  called  urea 
(CON2H4),  which  contains  the  nitrogen.  Although  the  urea 
is  solid  under  all  ordinary  conditions,  it  is  dissolved  in  the  liq- 
uids of  the  body,  since  it  is  soluble  in  water,  and  is  therefore  in  a 
state  of  solution  while  in  the  body.  These  three  waste  products 
are  not  only  valueless  but  distinctly  harmful,  and  it  is  necessary 
for  the  body  to  get  rid  of  them.  The  series  of  chemical  changes 
which  finally  results  in  waste  products  is  called  metabolism. 

Excretions. — The  elimination  of  the  waste  products  of  me- 
tabolism is  known  as  excretion.  The  carbon  dioxid  gas  passes 
into  the  blood,  and  when  the  blood  reaches  the  lungs  the  gas 
diffuses  from  the  blood  into  the  air.  The  waste  water  also  passes 
into  the  blood  and  is  passed  off  from  the  body  through  the  kid- 
neys, the  lungs,  and  the  skin.  The  urea  finds  its  way  into  the 
blood,  and  as  the  blood  flows  through  the  kidneys  (Fig.  92), 
they  take  the  urea  from  it.  They  then  pass  it  through  their 
ducts  dissolved  in  the  urine,  and  it  goes  to  the  bladder  and 


THE  PHYSIOLOGY  OF  AN  ANIMAL  211 

then  passes  to  the  exterior  with  the  faeces,  the  one  cloacal 
opening  serving,  in  the  frog,  for  the  exit  of  undigested  food 
as  well  as  for  the  urine. 

Support. — The  skeleton  serves  to  support  the  softer  part  of 
the  body. 

Motion. — The  motion  of  the  frog  is  accomplished  by  the 
muscles.  The  muscles  are  numerous,  and  each  has  its  own 
special  attachment  to  the  bones;  Fig.  89.  Every  muscle  pos- 
sesses the  power  of  shortening,  but  has  no  other  function; 
and  the  ordinary  muscles  are  attached  to  two  bones  in  such  a 
way  that  when  the  muscle  shortens  one  bone  is  moved  upon 
another.  All  the  motions  of  the  body  are  produced  by  the  short- 
ening of  the  different  muscles.  Many  of  the  muscles  are  in 
pairs,  one  of  each  pair  serving  to  bend  a  joint,  the  flexor,  and 
the  other  straightening  it,  the  extensor.  The  details  of  their 
actions  we  cannot  consider  here,  but  it  will  readily  be  seen  that 
with  the  many  muscles  present  in  the  frog's  body  a  great  variety 
of  motions  can  be  produced.  The  selection  of  the  proper  mus- 
cles to  produce  any  desired  motion  is  a  complicated  process, 
some  motions  indeed  requiring  the  orderly  selection  of  a  large 
number  of  muscles,  which  must  act  together  in  perfect  harmony. 
This  power  of  selecting  the  muscles  and  causing  them  to  act  in 
unison  and  in  harmony  with  each  other  is  called  coordination. 

The  Coordinating  System. — The  nervous  system  of  the  frog 
controls  all  other  functions.  As  already  seen,  it  consists  of  (1)  a 
central  system,  the  brain  and  spinal  cord;  (2)  the  peripheral 
system,  the  latter  composed  of  the  nerves  distributed  over  the 
body,  and  the  various  end  organs  of  the  nerves.  The  central 
system  is  really  the  center  of  activity,  and  the  nerve  fibers  are 
merely  paths  for  conducting  impulses  from  one  part  of  the  body 
to  another.  Some  of  the  end  organs  at  the  outer  ends  of  the 
nerves  receive  impulses  from  the  brain;  others  receive  them  from 
the  exterior  and  transmit  them  to  the  nerves  to  be  carried  to  the 
brain.  The  brain  corresponds  to  the  central  station  of  a  telephone 
system,  which  is  connected  with  all  parts  of  the  city  by  wires  hav- 


212  BIOLOGY 

ing  at  their  ends  the  individual  telephones  which  may  receive  com- 
munications from  the  central  system  or  send  messages  to  it. 
So  the  central  nervous  system  contains  the  intelligent,  originat- 
ing force,  and  being  in  communication  with  every  part  of  the 
body,  controls  all  of  the  functions  in  such  a  way  that  they  act  in 
harmony.  This  central  system  has  a  series  of  efferent  nerves, 
by  which  it  sends  messages  outward,  and  a  series  of  afferent 
nerves,  by  which  messages  are  brought  inward  to  the  brain. 
The  most  important  of  the  latter  are  the  sensory  nerves. 

Sense  organs. — Each  sensory  nerve  ends  in  a  sense  organ,  so 
formed  that  it  is  excited  by  definite  external  stimuli.  One  of 
them,  the  ear,  is  stimulated  by  vibrations  of  the  air;  another, 
the  eye,  by  vibrations  of  ether;  others  by  a  slight  pressure  or 
touch;  others  by  heat;  others  again,  by  chemical  substances, 
producing  taste;  and  others  by  vapors  in  the  form  of  gases, 
causing  the  sense  of  smell.  Figure  96  shows  the  microscopic 
structure  of  some  of  these  sensory  end  organs.  In  each  case 
the  end  organ  is  started  into  activity  by  an  external  stimulus, 
and  when  thus  excited  an  impulse  starts  from  it  over  the  nerve 
fiber  and  passes  to  the  central  part  of  the  nervous  system.  In 
the  central  system,  the  stimulus  produces  what  we  call  a  sensa- 
tion, and  this  gives  the  brain  a  knowledge  of  what  is  going  on 
at  the  outer  end  of  the  nerve.  Sensation  never  occurs  until  the 
impulse  reaches  the  brain.  From  these  sensations  the  brain 
obtains  information  as  to  what  is  going  on  in  different  parts  of 
the  body,  and  upon  this  information,  bases  its  knowledge  and 
regulates  the  activities  of  the  body. 

Reflexes. — The  nervous  system  is  made  up  of  a  mass  of 
neurons  whose  connections  with  each  other  are  inconceivably 
complex.  These  neurons,  with  their  long  axons,  unite  in  har- 
monious activity  the  different  organs  of  the  body,  and  they  do 
this  by  virtue  of  the  fact  that  their  axons,  though  distributed 
all  over  the  body,  all  converge  in  the  central  system,  where  they 
can  be  associated  together  by  the  numerous  neuron  bodies  that 
compose  these  central  ganglia;  Fig.  85.  The  courses  taken  by 


THE  PHYSIOLOGY  OF  AN  ANIMAL 


213 


these  impulses  after  reaching  the  centers  are  complex  in  the 
extreme,  and  quite  beyond  our  power  to  follow.  They  are  ac- 
companied by  sensations  and  by  whatever  of  consciousness 
the  animal  possesses,  and  they  * 

control  the  life  and  motions  of 
the  animal.  The  simplest  of 
these  connections  may  produce 
motion  without  any  conscious- 
ness on  the  part  of  the  animal. 
This  is  shown  diagrammat- 
ically  in  Figure  103.  Some  ex- 
ternal stimulus  excites  one  of 
the  sense  organs  in  the  skin,  s, 
and  starts  an  impulse  in  the 
nerve  fiber,  which  then  travels 
quickly  through  the  axon  to  its 
inner  end,  c,  in  the  cord.  The 
impulse  then  passes  out  of  the 
axon  through  the  arborizations, 
at  c,  to  the  neighboring  den- 
drites  of  other  neurons.  These 
neurons  may  be  motor  cells,  m, 
by  which  is  meant  that  their 


axons,  e,  extend  outward  and 


FlG.    103. A   DIAGRAM    ILLUSTRAT- 
ING   A    REFLEX    ACTION 


terminate  in  muscle  fibers,  as 

at  mu.    Hence  the  impulse  that 

enters  them,  after  passing  out 

over    the    axon,    eventually 

reaches  a   muscle   fiber,    and 

causes  the  muscle  to  contract. 

Such  an  action  may  take  place 

without  the  impulse  going  to  the  brain,  and  would  therefore 

not  involve   any  consciousness    or    any  sensation,   for  these 

latter  functions  occur  in  the  brain  only.     Hence  the  animal 

might  move  if  touched  by  an  irritating  object,  without  any 


An  impulse  that  starts  from  the  sense 
organs  in  the  skin,  s,  passes  to  the  spinal 
cord  through  the  afferent  nerve,  a.  Upon 
reaching  the  center,  at  c,  the  impulse  may 
pass  over  to  the  motor  cell,  TO,  from  whence 
it  passes  downward  through  the  efferent 
nerve,  e,  to  the  muscle  fibers,  mu.  Part 
of  the  impulse  from  the  c  may  pass  up 
through  the  fiber,  a,  to  the  brain  and  pro- 
duce sensation. 


214  BIOLOGY 

necessary  consciousness  on  its  part,  as  actually  happens  in 
sleep,  for  example.  Such  an  action  is  called  a  reflex  act,  a 
name  derived  from  the  idea  formerly  held  that  the  impulse 
starting  from  the  sense  organ  was  simply  reflected  back  after 
reaching  the  cord.  Although  we  know  to-day  that  the  impulse 
is  not  simply  reflected  back,  but  is  profoundly  modified  in  the 
cord,  the  name  reflex  is  still  retained  for  this  type  of  reaction. 

Although  a  reflex  act  is  not  necessarily  accompanied  by  con- 
sciousness or  sensation,  this  is  not  always  the  case.  From  the 
diagram  (Fig.  103),  it  is  evident  that  the  impulse,  on  its  arrival 
in  the  cord,  may  not  all  pass  into  the  motor  nerve  cell,  but  some 
of  it  may  pass  up  through  the  fiber,  a,  toward  the  brain,  and  this 
part  of  the  impulse,  when  it  reaches  the  brain,  will  give  rise 
to  a  sensation.  The  action  that  follows  might  still  be  the 
reflex,  or  it  might  be  a  truly  voluntary  one,  started  by  the  brain 
as  the  result  of  the  sensation.  Reflexes  play  a  very  large  part 
in  the  life  of  all  animals.  Even  in  our  own  life  many  of  our 
actions  are  thus  reflexly  performed  without  any  special  volition. 

Reproduction. — The  eggs  of  the  frog  are  only  developed  at 
certain  seasons  of  the  year.  Late  in  the  spring  and  early  in  the 
summer  the  ovaries  are  small,  but  toward  the  end  of  summer 
and  in  the  fall  the  eggs  begin  to  develop  and  cause  the  ovaries 
to  expand  until  they  almost  fill  the  body  cavity.  When  the 
frog  goes  into  the  dormant  condition  of  hibernation  (Lat. 
hibernare  =  to  pass  the  winter),  the  female  is  usually  greatly 
distended  with  the  swollen  ovary,  and  in  this  condition  the 
winter  period  is  passed.  The  oviducts  have  also  enlarged  and 
elongated,  and  remain  so  during  the  winter,  while  the  animal 
is  buried  under  ground.  With  the  opening  of  the  spring  the  frog 
emerges  and  resumes  its  active  life,  and  in  a  few  weeks  reaches 
what  is  called  the  breeding  season,  which  means  the  season 
for  the  discharge  of  the  sexual  products.  As  this  season  ap- 
proaches the  eggs  break  out  of  the  ovary  and  fall  into  the 
abdominal  cavity.  The  funnel-shaped  opening  of  each  oviduct 
is  provided  with  vibratile  cilia  (Fig.  100),  and,  probably  by 


THE  PHYSIOLOGY  OF  AN  ANIMAL  215 

their  action,  the  eggs  are  swept  into  the  opening,  and  then 
slowly  pass  down  through  the  coils  of  the  oviduct  toward  the 
uterus.  As  they  pass  along  they  are  covered  with  a  gelati- 
nous substance,  which  is  secreted  from  the  glands  in  the 
walls  of  the  ducts  and  forms  a  layer  around  the  eggs.  When 
the  eggs  reach  the  uterus  they  are  stored  there  for  a  time  until 
the  animal  is  ready  to  lay  her  eggs. 

With  the  approach  of  the  breeding  season  the  spermaries  of 
the  male  also  become  very  active  and  secrete  sperm  fluid.  This 
passes  down  the  ducts  to  be  stored  in  the  seminal  vesicles,  where 
it  remains  until  the  period  of  copulation. 

At  the  breeding  season  the  male  frog  fastens  himself  to  the 
female,  who  is  about  to  lay  her  eggs,  and  remains  firmly  attached 
to  her  until  she  lays  them,  remaining  thus  attached  for  days  or 
even  for  weeks  in  some  cases  (copulation).  After  the  eggs  are 
laid  the  male  leaves  the  female  and  pays  no  further  attention 
to  her.  When  the  eggs  are  laid  they  are  rather  slowly  passed 
from  the  body  by  the  cloacal  opening,  and  at  the  same  time 
the  male  ejects  the  sperm  fluid  from  his  body  over  them.  The 
sperms  themselves  penetrate  the  jelly  and  eventually  enter  the 
eggs,  producing  fertilization.  After  the  eggs  are  thus  laid  the 
ovaries  and  the  oviducts  contract  and  in  a  short  time  shrivel  to 
a  size  much  smaller  than  that  which  they  had  at  the 'breeding 
season.  This  diminished  size  continues  until  late  in  the  summer, 
when  the  ovaries  begin  to  increase  in  size  again  with  the  growth 
of  the  ova,  in  preparation  for  the  next  breeding  season. 

The  eggs  of  the  common  frog  are  always  laid  in  water  and 
at  first  form  a  rather  small  mass  of  eggs  with  their  surrounding 
j  elly .  But  the  j  elly  quickly  absorbs  the  water  and  swells  to  many 
times  its  original  size,  inclosing  each  egg  in  a  thick  layer.  This 
jelly  appears  to  have  two  purposes.  It  is  a  protection  to  the 
eggs  from  the  attack  of  birds  and  perhaps  other  enemies.  It 
seems  also  to  have  the  power  of  absorbing  the  sun's  rays  and 
holding  them  back  from  too  great  radiation,  the  result  being  that 
the  egg  is  kept  warmer  than  it  would  be  without  the  jelly.  This 


216  BIOLOGY 

hastens  the  development,  since  its  rate  is  dependent  on  tem- 
perature. Our  common  frog  lays  its  eggs  in  irregular  masses, 
which  may  be  found  in  abundance  in  the  spring  months  around 
pools  of  fresh  water.  The  toad  has  somewhat  similar  breeding 
habits,  but  lays  its  eggs  in  long  strings.  Inside  the  jelly  the 
fertilization  of  the  eggs  is  completed  and  the  development 
begins,  and  here  the  young  remain  until  they  are  ready  to  hatch 
as  young  larvae. 

PHYSIOLOGY  OF  THE  EARTHWORM 

The  organs  of  the  earthworm  are  much  simpler  than  those  of 
the  frog.  Some  of  the  systems  of  organs  found  in  the  frog  are 
apparently  absent  in  the  earthworm.  There  are,  for  example, 
no  lungs  nor  other  special  organs  devoted  to  respiration;  there 
is  neither  heart  nor  system  of  bones  for  support.  But  although 
some  of  these  systems  of  organs  appear  to  be  absent,  their 
functions  are  not  lacking.  In  other  words,  the  earthworm  has 
exactly  the  same  functions  of  life  as  the  frog,  but  carries  them 
out  in  a  simpler  way.  Respiration  is  carried  on  through  the 
skin;  the  motions  of  the  animals  are  confined  to  a  writhing 
motion  made  by  the  muscles  of  the  body  wall ;  the  circulation 
of  the  blood  is  produced  by  the  contraction  of  the  blood  vessels 
instead  of  by  a  heart;  excretions  are  carried  on  through  the  skin 
and  also  by  the  nephridia.  In  short,  the  earthworm  has  the 
same  general  functions  as  the  frog,  only  they  are  carried  out  on 
a  simpler  scale  and  by  a  simpler  series  of  organs.  Since  its 
organs  are  simpler,  we  speak  of  the  earthworm  as  having  a 
lower  organization  than  the  frog. 


CHAPTER  XI 

THE  DIFFERENCES  BETWEEN  ANIMALS  AND  PLANTS: 
THE  MUTUAL  RELATIONS  OF  ORGANISMS 

THE  DIFFERENCES  BETWEEN  ANIMALS  AND  PLANTS 

IF  we  confine  our  attention  to  the  larger  organisms,  the 
differences  between  plants  and  animals  are  very  evident;  but 
when  we  turn  our  attention  to  some  of  the  lower  members  of 
each  group,  the  differences  are  less  evident  and  most  of  them 
disappear.  A  castor  bean  and  a  frog  are  very  unlike,  but 
Peranema  and  Euglena  (Fig.  29)  are  so  similar  that  it  is 
hardly  possible  to  say  whether  either  of  them  is  an  animal 
or  a  plant. 

In  their  life  functions,  too,  the  higher  plants  and  animals 
differ  widely.  Most  of  the  general  functions  of  animal  life 
are  possessed  in  a  modified  form  by  plants  also;  but  since 
some  functions  are  possessed  by  animals  alone,  a  division  of 
functions  into  two  categories  is  frequently  adopted. 

Vegetative  functions  are  those  possessed  by  both  animals 
and  plants.  They  are  chiefly  associated  with  food  and  growth, 
and  are:  alimentation,  circulation,  respiration,  excretion,  and 
reproduction. 

Animal  functions  are  those  possessed  by  animals  and  not 
by  plants.  They  are  motion  and  coordination. 

Both  animals  and  plants  have  vegetative  functions,  but  they 
are  carried  on  quite  differently  in  the  two  groups,  resulting  in 
a  radically  different  type  of  life  in  animals  and  plants.  The 
study  already  made  of  the  biology  of  organisms  enables 
us  now  to  ask  intelligently,  What  is  the  difference  be- 
tween animals  and  plants?  Although  it  is  fairly  easy  to  see 
the  difference  between  a  tree  and  a  dog,  when  we  come  to 
extend  the  comparison  to  smaller  and  lower  organisms  it 
becomes  more  and  more  difficult  to  determine  any  distinc- 

217 


218  BIOLOGY 

tions  between  the  two  kingdoms.  Indeed,  when  we  analyze 
the  subject  to  its  limit,  we  find  it  impossible  to  draw  any  sharp 
line  separating  animals  and  plants,  for  there  are  some  living 
things  which  show  so  few  characteristics  of  either  kingdom 
that  we  cannot  determine  with  accuracy  whether  they  belong 
to  one  group  or  the  other.  It  is  possible,  however,  to  draw  a 
general  distinction  between  the  two,  and  from  this  general 
distinction  we  can  derive  certain  other  secondary  differences, 
which  are  more  evident. 

The  Fundamental  Distinction. —  The  primary  distinction  be- 
tween animals  and  plants  is  in  the  process  of  photosynthesis. 
The  plant  kingdom  alone  has  the  power  of  utilizing  the  rays 
of  the  sun  and  manufacturing  starch  out  of  carbon  dioxid  and 
water:  animals  never  have  this  power.  From  this  primary 
distinction  arise  several  other  minor  points  of  difference,  more 
or  less  sharply  separating  these  two  groups. 

Secondary  Differences. —  A .  Color. —  Plants  which  have  the 
power  of  photosynthesis  are  provided  with  the  green  coloring 
matter,  chlorophyll.  Animals,  on  the  other  hand,  are  not  pro- 
vided with  this  coloring  matter. 

B.  Motion. —  Since  animals  live  upon  solid  foods,  they  have 
to  search  for  it,  and  they  are,  as  a  rule,  provided  with  motion. 
Plants,  on  the  other  hand,  having  no  need  to  search  for  their 
food,  since  they  find  it  in  the  atmosphere  and  soil,  have  not, 
as  a  rule,  developed  the  power  of  motion. 

The  various  methods  of  motion  developed  by  animals  may 
be  summarized  as  follows:  (1)  Amoeboid  movement,  as  found 
in  Amoeba,  by  means  of  lobes  of  the  living  protoplasm.  It  is 
confined  to  unicellular  organisms.  (2)  Ciliated  and  flagellated 
motion,  produced  by  vibratile,  hairlike  processes  of  the  proto- 
plasm. Cilia  are  moderately  short  processes,  and  where  found 
are  usually  present  in  large  numbers.  They  are  found  in 
many  unicellular  animals  and  also  in  multicellular  forms. 
Even  the  highest  animals  have  cilia  on  the  cells  lining  the  air 
passages  and  various  other  ducts.  Flagella  are  longer  than 


DIFFERENCES  BETWEEN  ANIMALS  AND  PLANTS      219 

cilia,  and  occur  only  in  small  numbers  on  any  cell,  one  or  two 
being  the  usual  number.  Higher  animals  do  not  have  true 
flagella,  except  in  their  sperms;  see  page  250.  (3)  Muscular 
Movements. — In  all  animals  above  the  unicellular  forms  cer- 
tain cells,  or  parts  of  cells,  become  specially  modified  for  con- 
traction, thus  becoming  muscles.  These  develop  into  a  system 
which  produces  the  many  types  of  locomotion  possessed  by 
animals. 

While  plants  as  a  rule  are  stationary,  a  few  of  them  possess 
independent  motion.  Spores  of  many  plants  possess  flagella 
or  cilia;  some  of  the  lowest  show  amoeboid  motion,  and  some 
have  methods  of  motion  not  yet  understood,  like  Diatoms  and 
Oscillaria;  Fig.  68.  Among  higher  plants  movements  of  different 
parts  of  the  leaves,  stamens,  etc.,  are  not  uncommon.  No 
muscles  are  developed,  however,  in  plants,  the  motions  being 
due  to  slow  changes  in  the  protoplasm,  which  are  not  well 
understood.  An  independent  locomotion  is  unknown  among 
any  plants  except  those  of  the  lowest  orders. 

C.  Sensitiveness. —  In  order  to  distinguish  their  food,  ani- 
mals have  developed  sensitiveness  and  sensations.    Plants  not 
needing  to  distinguish  food  so  accurately  have  not  developed 
much  sensitiveness. 

D.  Structure. —  As  a  rule   animals  have  their  bodies  con- 
densed into  a  small  compass,  and  are  provided  with  an  opening 
for  taking  in  food, —  the  mouth, —  which  is  connected  with  a 
digestive  system.     Typical  plants,  since  they  feed  upon  gases 
and  water,  which  are  distributed  everywhere,  have  their  bodies 
widely  expanded  into  branches,  leaves,  and  root  hairs,  in  order 
to  come  in  contact  with  a  large  amount  of  air  and  soil.    They 
never  have  any  mouths,  since  they  do  not  take  solid  food, 
and  consequently  have  no  digestive  system. 

The  Income  and  Outgo  of  Animals  and  Plants. —  An  animal 
has  an  income  as  follows :  — 

Proteids,  obtained  from  animal  or  vegetable  food,  but  all 
originally  derived  from  green  plants. 


220  BIOLOGY 

Hydrocarbons  (fats),  derived  from  both  animal  and  vege- 
table food. 

Carbohydrates,  derived  chiefly  from  vegetable  foods. 

Water. 

Oxygen,  taken  from  the  air  by  the  respiratory  organs  into 
the  blood. 

Salts,  of  various  kinds  in  the  foods. 

The  outgo  of  an  animal  consists  of:  — 
Carbon  dioxid,  excreted  from  the  respiratory  organs. 
Water,   excreted  from  the  skin,   kidneys,   and  some  other 
organs. 

Urea,  excreted  by  the  kidneys  or  their  equivalents. 
Proteids,  eliminated  in  the  reproductive  bodies. 
Salts,  in  various  excretions. 

After  an  animal  has  reached  its  full  growth,  the  income  and 
the  outgo  practically  balance.  With  some  animals  this  period 
of  equilibrium  lasts  a  long  time,  perhaps  for  years.  With 
others,  growth  may  continue  until  death  comes,  in  which  case 
there  is  never  any  period  of  actual  balance. 

The  income  of  a  plant  consists  of:  — 
Carbon  dioxid,  from  the  air. 
Water,  from  the  soil. 
Minerals,  from  the  soil. 

The  outgo  of  a  plant  consists  of:  — 
Oxygen,  from  the  leaves. 
Water,  from  the  leaves. 

Carbon  dioxid,  from  the  leaves  and  other  parts. 
Proteids  and  various  other  substances,  eliminated  with  dead 
leaves,  branches,  seeds,  and  other  reproductive  bodies. 

No  Sharp  Distinction  between  Animals  and  Plants. — The 
criteria  above  given  are  ordinarily  sufficient  to  distinguish 
between  animals  and  plants,  and  will  separate  typical  forms; 


DIFFERENCES  BETWEEN  ANIMALS  AND  PLANTS      221 

but  when  we  come  to  consider  low  types,  some  or  all  of  these 
distinctions  disappear.  There  are,  for  example,  many  plants 
which  have  no  chlorophyll  (molds,  toadstools,  etc.),  and  hence 
have  no  power  of  photosynthesis;  but  they  are,  nevertheless, 
clearly  plants,  for  no  one  would  for  an  instant  think  of  con- 
fusing them  with  animals,  even  though  they  do  not  contain 
chlorophyll.  Some  plants  have  independent  motion,  while 
some  animals  are  stationary.  Some  plants  are  sensitive.  The 
distinction  of  shape  applies  only  to  the  higher  organisms;  for 
among  the  microscopic  forms  no  distinction  can  be  seen  be- 
tween the  shape  of  animals  and  plants,  some  animals  having 
no  mouth,  and  some  plants,  as  well  as  animals,  having  their 
bodies  condensed  rather  than  expanded.  Thus  it  appears 
that  each  of  the  distinguishing  characters  separating  animals 
and  plants  breaks  down  when  we  come  to  apply  it  closely  to 
some  of  the  low  forms  of  life ;  until  we  have  to  admit  that  there 
is  no  absolute  criterion  separating  the  two  kingdoms. 

Nevertheless,  there  is  rarely  any  real  difficulty  in  making 
the  distinction.  It  is  true  that  there  is  a  difference  of  opinion 
as  to  whether  a  few  of  the  very  low  forms  should  be  called 
animals  or  plants;  but  when  we  take  all  of  the  above  facts  into 
consideration,  it  is  only  in  a  few  instances  that  we  are  unable 
to  say  positively  that  any  given  organism  is  either  animal  or 
plant.  Most  of  the  difficulty  is  confined  to  the  microscopic 
forms  which  are  among  the  lowest  organisms,  and  the  fact  that 
among  these  there  is  no  absolutely  fixed  line  between  the  two 
kingdoms  is  of  special  significance  as  suggesting  the  origin  of 
the  two  kingdoms  from  a  common  starting  point  by  a  process 
of  evolution. 

Organisms  which  possess  chlorophyll,  and  consequently  nour- 
ish themselves  by  photosynthesis,  are  sometimes  said  to  be  holo- 
phytic  (Gr.  holos  =  whole  +  phyton  =  plant) .  In  contradistinc- 
tion, organisms  which  have  no  chlorophyll  and  must  depend 
upon  others  for  sustenance  are  called  holozoic  (Gr.  zoon  =  ani- 
mal). Animals  are  practically  all  holozoic,  and  green  plants 


222  BIOLOGY 

holophytic;  but  many  plants  are  holozoic,  a  condition  which  is 
true  of  all  Fungi. 

Protozoa  and  Protophyta. —  Both  plants  and  animals  may 
be  found  among  unicellular  organisms,  the  unicellular  animals 
being  known  as  PROTOZOA  (Gr.  protos  =  first  +  zoon  =  ani- 
mal), and  unicellular  plants  as  PROTOPHYTA  (Gr.  phyton  = 
plant) ;  see  Chapter  III.  Among  such  organisms  there  is  some- 
times a  difficulty  in  distinguishing  between  animals  and  plants, 
since  any  structure  of  a  distinctive  character  is  lacking.  Even 
here,  however,  the  majority  of  forms  group  themselves  in 
one  of  the  two  kingdoms,  so  that  they  can  readily  be  separated. 
There  are,  however,  a  few  forms  which  prove  a  puzzle.  Euglena 
(Fig.  29  B)y  for  example,  has  green  chlorophyll,  and  is  thus  allied 
to  the  vegetable  kingdom  (holophytic);  but  it  has  also  the  power 
of  motion,  a  mouth,  and  a  red  eye-spot.  Peranema  (Fig.  29  A)t 
however,  which  is  clearly  allied  to  Euglena,  has  no  chlorophyll 
and  no  plant  characters  (holozoic).  We  may,  with  equal  justice, 
call  both  animals  or  both  plants,  or  perhaps  one  an  animal  and 
one  a  plant.  The  bacteria  (Figs.  33-35)  represent  another  group 
which  has  been  difficult  to  classify  clearly ;  and  for  many  years 
after  they  were  first  studied  there  was  a  considerable  difference  of 
opinion  as  to  where  they  belonged.  They  have  a  method  of 
life  much  like  that  of  animals,  but  their  general  structure, 
their  multiplication,  their  division  to  form  long  chains,  and  an 
occasional  formation  of  spores,  are  points  much  more  like 
plants,  especially  the  Fungi.  Continued  study  of  the  organisms 
has  finally  led  to  the  conclusion  that  bacteria  must  be  regarded 
as  plants  rather  than  animals,  associated  with  the  group  of 
Fungi,  and  considered  as  resembling  yeast  and  molds. 

A  few  such  organisms  as  these  are  the  only  ones  that  present 
much  difficulty  in  distinguishing  between  animals  and  plants, 
and  even  these  can  be  called  animals  or  plants  with  a  consid- 
erable degree  of  certainty.  While  no  sharp  line  can  be  drawn, 
the  difficulty  of  separating  them  is  really  not  very  great,  and 
even  among  unicellular  forms  it  is  rare  that  we  cannot  deter- 


DIFFERENCES  BETWEEN  ANIMALS  AND  PLANTS     223 


mine  satisfactorily  whether  to  call  an  organism  an  animal  or 
a  plant. 

Metazoa  and  Metaphyta. —  With  the  multiplication  of  cells 
and  their  differentiation  we  find  that  the  distinction  between 
animal  and  plant  at 
once  becomes  marked, 
until  there  is  no  longer 
any  similarity  between 
them.  Indeed,  in  all 
organisms  above  the 
Protozoa  and  the 
Protophyta,  the  two 
kingdoms  are  sharply 
separated,  all  multi- 
cellular  organisms  be- 
ing either  so  distinctly 
like  plants  or  like  ani- 
mals that  the  difficulty 
of  distinguishing  them 
disappears  entirely. 
From  this  point  up, 
plants  usually  not  only 
possess  chlorophyll, 
but  also  show  a  gen- 
eral structure  which 
indicates  that  they 
are  adapted  for  the 
absorption  of  gases 
from  the  air,  of  water 
from  the  soil,  and  for 

the  purpose  of  carrying  on  the  process  of  photosynthesis;  while 
animals  have  a  structure  of  body  adapted  for  taking  only  solid 
or  liquid  food.  The  difference  in  the  shape  of  the  animal 
and  plant  body  becomes  so  well  marked  that  there  is  no  longer 
any  confusion  between  them.  Even  though  we  find  large 


FIG.  104. — DROSERA,  A  CARNIVOROUS  PLANT 

Small  insects  are  captured  by  the  hairs  on  the  leaves, 
digested,  and,  in  a  measure,  assimilated  by  the  plant. 


224  BIOLOGY 

groups  of  plants  that  have  quite  lost  their  chloropnyll  (toad- 
stools, molds,  etc.),  there  is  no  longer  any  difficulty  in  deter- 
mining that  they  are  to  be  grouped  with  plants  rather  than 
with  animals,  in  spite  of  their  not  having  any  green  color- 
ing matter.  When,  too,  we  find  a  plant  like  the  sundew  (Fig. 
104),  which  captures  insects  by  means  of  the  hairs  on  its  leaves, 
and  digests  and  assimilates  them,  we  call  it  a  "carnivorous 
plant"  (Lat.  caro  (carnis)  =  flesh  +  vorare  =  to  eat),  but  do  not 
confound  it  with  animals.  The  Metazoa  (Gr.  meta  =  after  -f 
zoon  =  animal)  and  Metaphyta  (Gr.  phyton  =  plant)  are  sharply 
distinct. 

CONTRAST  BETWEEN  THE   ACTIVITIES   OF  ANIMALS  AND 

PLANTS 

The  similarities  and  differences  between  animals  and  plants 
may  be  better  understood  if  their  properties  are  contrasted 
with  each  other  in  regular  order.  The  following  contrasts 
illustrate  the  distinction  between  these  two  groups :  — 

1.  Alimentation. —  In    animals    this    system    consists    of    a 
mouth,  stomach,  intestine,  and  digestive  glands;  food  is  taken 
into  the  body  either  as  a  solid  or  a  liquid.    In  plants  the  system 
is  poorly  developed,  consisting  of  root  hairs  for  taking  in  liq- 
uids, and  stomata  for  absorbing  gases,  but  having  no  digestive 
organs.     The  foods  absorbed  are  either  liquids  or  gases,  but 
never  solids. 

2.  Circulation. —  In  animals  circulation  is  brought  about  by 
a  heart  and  blood  vessels,  or  something  corresponding  to  them. 
In  plants  the  water  absorbed  from  the  roots  ascends  the  stem, 
and  passes  out  into  the  leaves  by  a  process  known  as  the  ascent 
of  sap,  and  the  materials  formed  in  the  leaves  are  dissolved 
and  eventually  diffused  throughout  the  plant,  passing  down- 
ward in  certain  of  the  cells  of  the  stem.     There  are  no  real 
blood  vessels,  no  heart,  no  blood,  and  no  definite  circulation. 

3.  Metabolism. — In  animals  metabolism  is  essentially  destruc- 
tive.   The  animal  uses  as  food  organic  compounds  like  carbo- 


DIFFERENCES  BETWEEN  ANIMALS  AND  PLANTS     225 

hydrates,  fats,  and  proteids;  combines  them  with  oxygen,  and, 
as  a  result,  produces  as  waste  products  carbon  dioxid,  water, 
and  urea.  The  foods  are  broken  to  pieces,  and  the  energy 
thus  liberated  is  utilized;  see  Chapter  XV.  In  plants  the  proc- 
ess is  primarily  constructive,  but  there  is  in  plant  life  both 
a  constructive  and  a  destructive  metabolism.  By  the  former 
the  plant  uses  carbon  dioxid,  water,  and  nitrates,  which 
are  combined  in  the  plant  to  form  organic  substances,  like 
starches,  proteids,  etc.,  and  in  the  combination  solar  energy 
is  stored  away.  As  an  excretion,  there  are  produced  oxygen 
and  water.  The  destructive  process  of  plants  is  essentially  like 
that  of  animals:  the  compounds  built  up  by  the  first  process 
are  destroyed  by  the  second.  The  total  amount  of  construction 
in  green  plants  is  greater  than  the  amount  of  destruction, 
and  therefore  the  green  plants  store  away  organic  products 
which  may  subsequently  be  utilized  by  plant  life. 

4.  Respiration. —  Animals  usually  have  lungs  or  gills  filled 
with  blood ;  they  always  absorb  oxygen,  and  eliminate  the  car- 
bon dioxid.     In  plants  the  respiration  is  carried  on  through  the 
stomata  of  the  leaves;  when  carrying  on  photosynthesis,  plants 
absorb  carbon  dioxid  and  eliminate  oxygen;  when  not  carry- 
ing on  photosynthesis,  the  gas  absorbed  is  oxygen  and  the  gas 
liberated  is  carbon  dioxid. 

5.  Excretion. —  In  animals  carbon  dioxid   is  excreted  from 
the  lungs,  water  from  the  skin  and  kidneys,  and  urea  from 
the  kidneys.     In  plants  there  is  no  well-developed  excretory 
system,    although   gases   are   excreted   through   the   stomata, 
and  certain  other  substances  may  pass  out  through  the  bark 
or  through  the  roots  into  the  soil. 

6.  Motion. —  The  muscles  of  animals  develop  a  high  degree 
of  motion.    In  plants  motion  is  very  rarely  developed,  although 
it  is  not  wholly  lacking,  some  plants  being  well  supplied  with 
motile  power.    They  do  not,  however,  have  muscles;  and  when 
they  have  motion,  they  use  other  forms  of  mechanism. 

7.  Support. —  Animals  usually  have  a  skeleton  of  shell  or 


236  BIOLOGY 

bone,  either  internal  or  external.  In  plants  the  supporting 
structure  is,  as  a  rule,  developed  better  than  in  animals,  and 
consists  of  the  great  mass  of  wood  or  other  resisting  material 
found  throughout  the  plant. 

8.  Coordination. —  All  animals,  except  the  unicellular  forms, 
have  a  nervous  system,  usually  centering  in  the  brain,  which 
brings  into  coordination  the  various  functions  of  life.    In  plants 
there  is  no  coordinating  system  and  practically  no  coordination 
of  the  different  parts.    Each  part  of  the  plant  may  live  its  life 
to  a  considerable  degree  independently  of  the  others. 

9.  Reproduction. —  The  reproductive  processes  of  animals  and 
plants  are  very  similar.     Both  produce  eggs  and  sperms,  and 
have  a  sexual  reproduction;  and  in  both  there  may  be  repro- 
duction by  an  asexual  method,  although  in  animals  the  asexual 
reproduction  is  less  common  than  in  plants.    In  the  higher 
animals  the  power  of  asexual  reproduction  is  lost,  while  in 
even  the  highest  plants  the  process  of  asexual  reproduction  has 
commonly  been  retained.     In  the  higher  members  of  both 
groups,  sexual  reproduction  by  eggs  and  sperms  is  universal. 

THE   MUTUAL  RELATIONS   OF   ORGANISMS 

The  close  relation  of  organisms  to  each  other  is  evident, 
since  all  animals,  as  well  as  all  colorless  plants,  are  dependent 
upon  green  plants  for  their  food.  They  vary  greatly,  however, 
in  their  methods  of  obtaining  their  food. 

Food  Habits 

Plants  may  be  divided  into  three  groups,  according  to  their 
methods  of  obtaining  their  food :  — 

1.  Autophytes  (Gr.  autos  —  self  +  phyton  =  plant). —  Plants 
which  are  not  dependent  upon  organic  foods,  but  are  able  to 
take  care  of  themselves  by  subsisting  upon  the  minerals  from 
the  soil,  together  with  the  gases  from  the  air,  are  called  auto- 
phytes.  These  include  the  green  plants  (holophytic)  only;  and 
strictly  speaking,  even  these  plants  are  in  a  measure  dependent 


THE  MUTUAL  RELATIONS  OF  ORGANISMS  227 

upon  others.  The  minerals  that  they  absorb  from  the  soil 
are  available  for  plant  life  only  after  the  bacteria  and  other 
soil  organisms  have  acted  upon  them,  the  fertility  of  the  soil 
depending  upon  its  microscopic  life.  The  autophytes,  however, 
do  not  need  organic  food,  and  in  this  respect  are  much  more 
independent  than  the  other  two  groups. 

2.  Saprophytes    (Gr.  sapros  =  rotten  +  phyton  =  plant).  — 
Plants  which  feed  upon  the  dead  bodies  of  other  organisms  are 
saprophytes.     The  plants  usually  included   under  this  head 
are  the  Fungi*    These  constitute  the  scavengers  of  the  world, 
and  may  be  found  everywhere  in  the  soil  or  in  bodies  of  water, 
where  they  consume  whatever  excretions  of  animals  or  plants 
there  may  be;  or  live  upon  dead  roots,  leaves,  and  branches; 
they  live,  indeed,  upon  various  dead  materials  that  have  been 
derived  either  from  animal  or  plant  life.     Such  organisms  are 
almost  universally  distributed  over  the  earth,  and  they  cause 
all  decay  and  putrefaction,  these  two  processes  being  the  result 
of  the  destruction  of  organic  material  by  Fungi.     This  class 
of  organisms  is  ever  at  work  around  us,  consuming  the  bodies 
of  dead  animals  and  plants. 

3.  Parasites. —  Plants  which  live  upon  and  feed  upon  other 
living  organisms   are  parasites.     In  such   cases  we  call  the 
organism  upon  which  they  feed  the  host.     Parasitism  is  very 
common  among  both  plants  and  animals,  nearly  every  species 
having  special  parasites  feeding  upon  it.    As  a  rule,  the  para- 
sitic plants  lack  chlorophyll  and  belong  to  the  group  of  Fungi. 
Both  saprophytes  and  parasites  are  holozoic. 

Animals  have  similar  relations,  although  in  some  respects 
they  are  more  complicated.  No  animals  live  a  life  quite  inde- 
pendent of  organic  food,  like  the  autophytes,  since  they 
lack  chlorophyll.  The  great  majority  of  animals  are  called 
free-living,  but  they  feed  upon  dead  organic  material  (vegetable 
or  animal  food),  and  in  this  respect  resemble  saprophytes. 
Quite  a  large  number  of  animals  also  feed  upon  a  living  host, 
and  are  consequently  parasites. 

"Under  Fungi  are  included  bacteria. 


228 


BIOLOGY 


Symbiosis 

Among  both  animals  and  plants,  however,  we  not  infre- 
quently find  different  individuals  associated  and  living  in 
mutual  relations  which  may  or  may  not  be  those  of  parasite 
and  host.  The  term  symbiosis  (Gr.  sun  =  with  +  bios  =  life), 
which  may  refer  to  either  animals  or  plants  (literally  meaning 
living  together),  is  applied  to  a  variety  of  relations  where  two 
organisms  live  in  close  relation  to  each  other,  and  is  in  con- 
trast to  free-living  conditions  where  organisms  live  separately 
from  others.  The  purpose  of  symbiosis  is  not  always  the  same. 
Sometimes  it  is  to  the  mutual  advantage  of  both  members; 
sometimes  it  is  to  the  advantage  of  one  and  the  detriment  of 
the  other,  in  which  case  it  becomes  parasitism.  In  accordance 
with  the  relation  of  the  two  members  of  the  group,  svmbiosis 
may  be  divided  into  several  types  as  follows :  — 

Helotism. —  In  helotism  (Gr.  Helot  =  a  slave)  one  organism 
is  enslaved  by  the  other;  neither  is  especially  injured,  but 

one  does  the  work  for  the 
other.  Among  animals  the 
best  example  of  this  is 
among  the  slave-making 
ants,  where  one  species  of 
ants  makes  a  slave  of  an- 
other species,  the  slave  do- 
ing all  the  work  for  the 
slave-maker.  Both  indi- 
viduals carry  on  their  life 
in  satisfactory  fashion,  and 
neither  is  particularly  in- 
jured by  the  relation. 

Mutualism. — When  both 
members  of  a  group  obtain 
an  advantage  from  associa- 
tion, it  is  called  mutualism.     As   examples  of  this,  may  be 
mentioned   the  relation  between  domesticated    animals,   like 


sn 


FIG.  105. —  AN  EXAMPLE  OP 
MUTUALISM 

A  hermit  crab  cr,  lives  in  the  shell  of  the 
snail,  sn,  and  an  anemone,  an,  fastens  itself  to 
the  outside  of  the  shell.  Both  animals  are  bene- 
fited. 


THE  MUTUAL  RELATIONS  OF  ORGANISMS 


229 


dogs,  and  the  human  race.  Among  lower  animals  the  associa- 
tion of  a  hermit  crab  with  a  sea  anemone  is  an  illustration; 
Fig.  105.  Here  the  anemone  gains  an  advantage  from  being 
carried  to  and  fro,  while  the 
hermit  crab  is  protected  by 
the  nematocysts,  which,  as 
in  Hydra  (page  144),  are 
abundant  on  the  tentacles 
of  the  anemone,  and  which 
by  their  poison  protect  the 
crab  from  the  attack  of 
enemies. 

Mutualism  is  rather  more 
common  among  plants  than 
animals.  An  example  is  the 
common  gray  mosses  (Li- 
chens)  that  grow  on  rocks  FIG.  106.— CLADO NIA\  A  COMMON  LICHEN, 
or  tree  trunks;  Fig.  106.  GROWING  ON  ROCKS 

The  microscope  shows  that  ,  At  B  is  show,n  th.e  y°un?.  mycelium,  beginning 

to  grow  around  a  single  cell  of  the  green  alga. 

this  plant  is  a  combination 

of  a  fungus  and  a  green  plant;  Fig.  107.  In  this  association 

the  green  plant  carries  on 
photosynthesis,  furnish- 
ing starch  for  both  itself 
and  the  fungus;  on  the 
other  hand,  the  fungus 
furnishes,  for  the  benefit 
of  the  green  plant,  a 
lodging  place  and  a  con- 


siderable  quantity  01 
carbon  dioxid  and  water, 


FlG.    107. A   MAGNIFIED    SECTION   OF 

A   LICHEN 

Showing  that  it  is  made  up  of  a  fungus,  m,  and    which      it      Collects     from 
cells  of  a  spherical,  green  plant,  a. 


tion,  ^therefore,  seems  to  be  one  of  mutual  advantage.     An- 
other  example  is  a  group  of  bacteria  which  grows    within 


230  BIOLOGY 

the  little  nodules  on  the  roots  of  plants  like  peas  and  beans. 
If  the  roots  of  peas,  beans,  clover,  or  similar  plants,  be  care- 
fully removed  from  the  soil,  they  will  usually  be  found  covered 
with  little  nodules  ranging  in  size  from  the  head  of  a  pin  to  a 
large  pea.  These  are  found  to  be  produced  by  bacteria  which 
enter  the  roots  and  grow  and  multiply  in  their  tissues.  But 
the  association  is  mutually  advantageous.  The  bacteria  are 
useful  in  collecting  nitrogen  from  the  air  which  the  pea  utilizes 
for  its  own  benefit;  and,  on  the  other  hand,  the  bacteria  get 
the  benefit  of  a  lodging  place  and  nourishment  in  the  roots 
of  the  tubercle,  and  therefore  are  themselves  benefited  by  the 
association. 

Commensalism. —  In  commensalism  (Lat.  cum  =  with  + 
mensa  =  table)  the  two  organisms  live  together  without  notice- 
able advantage  or  disadvantage  to  either.  As  an  example, 
may  be  mentioned  the  small  crab  that  lives  in  the  oyster  shell, 
doing  no  injury  to  the  oyster  and  gaining  no  special  advantage. 
Various  vines  which  cling  to  trees  offer  another  example. 
Some  of  these  vines  force  their  rootlets  into  the  tissues  of  the 
tree  and  do  it  injury;  these  are  true  parasites.  But  other 
vines  simply  use  the  tree  for  the  support  of  their  weak,  climb- 
ing stem,  and  neither  plant  is  particularly  benefited  or  injured 
by  the  other,  except  that  the  vine  is  enabled  by  its  climbing 
habit  to  send  its  leaves  up  into  the  sunlight. 

Parasitism. —  In  parasitism  the  mutual  relationship  is  such 
that  one  individual  is  benefited  at  the  expense  of  the  other. 
The  host  is  always  injured,  while  the  parasite  is  benefited. 
Among  parasites  we  recognize  two  types. 

Ectoparasites. —  Parasites  that  live  upon  the  outside  of  their 
host  are  ectoparasites.  As  a  rule,  they  are  not  very  harmful, 
though  they  may  be  so.  Among  them  are  some  in  which  a 
parasitic  life  is  only  a  part  of  their  existence.  The  mosquitoes 
live  chiefly  upon  various  juices,  but  occasionally  suck  the  blood 
of  human  beings.  In  a  second  class,  like  the  bedbug,  the  animals 
live  wholly  upon  the  nutrition  from  their  host,  but  do  not 


THE  MUTUAL  RELATIONS  OF  ORGANISMS 


231 


attach  themselves  to  the  host  permanently.  A  third  type,  like 
the  lice,  lives  wholly  upon  its  host  and  has  no  life  apart  from  it. 
While  these  ectoparasites  may  be  trouble- 
some, they  are  not  especially  injurious, 
except  when  they  transmit  disease  germs. 
Endoparasites. —  Parasites  that  live 
within  the  body  of  the  host  are  endo- 
parasites.  They  are  numerous  and  pro- 
duce far  more  mischief  than  ectoparasites. 
Among  them  are  those  that  produce  vari- 
ous deadly  diseases  like  trichinosis  (Fig. 
108),  tuberculosis,  diphtheria,  etc. 

The  Effect  of  Parasitism 

Parasitism  occurs  among  both  animals 
and  plants.  The  number  of  species  of 
parasites  is  very  great,  but  cannot  be 
estimated.  Nearly  all  species  of  animals 
and  plants  have  their  own  parasites,  and 
some  have  several  species  of  parasites 
infesting  them.  For  this  reason  it  is 
sometimes  stated  that  there  are  at  least 
as  many  species  of  parasites  as  there 
are  species  of  non-parasitic  organisms. 
The  effect  of  the  parasitism  upon  both 
host  and  parasite  is  profound,  but  natu- 
rally quite  different. 

Upon  the  Host. — The  parasite  usually 
injures  the  host  and  is  then  spoken  of  as 
pathogenic  (Gr.  pathos  =  disease  +  -geneia 
=  producing).  The  amount  of  injury  varies  widely.  In  some 
cases,  the  parasite  produces  disease  and  even  the  death  of 
the  host.  Trichina  is  a  parasitic  worm  (Fig.  108),  which  occa- 
sionally causes  trichinosis  in  man,  resulting  sometimes  in  death. 
Certain  flies  occasionally  make  their  way  into  the  skull  cavities 


'A        B 

FIG.  108.— TRICHINA 

A,  a  single  worm  showing 
its  internal  anatomy;  B, 
worm  coiled  up  in  a  bit  of 
muscle  of  pork.  If  uncooked 
pork  containing  these  worms 
is  eaten  they  are  set  free  in 
the  intestines  and  a  case  of 
trichinosis  results. 


232 


BIOLOGY 


of  cattle,  producing  serious  and  fatal  brain  disease.  Malarial 
organisms  (Fig.  25)  live  as  parasites  in  hiiman  blood  and  pro- 
duce malaria.  Various  parasitic  bacteria  produce  serious  dis- 
eases in  man,  as  typhoid  fever,  tuberculosis,  diphtheria,  etc. 
The  same  is  true  of  plants.  The  various  wilts,  rusts,  and  blights 
are  serious  plant  diseases,  frequently  spreading  from  plant  to 
plant,  and  producing  death  and  destruction  of  the  host.  AH 
are  produced  by  parasites  growing  in  the  plant  tissues.  Fungi 
of  various  kinds  are  the  cause  of  the  greater  number  of  plant 


FIG.  109.— A  WILLOW 
LEAF  ATTACKED  BY 
"MILDEW"  CAUSED 
BY  A  PARASITIC 

FUNGUS 


FIG.  110.— THE  "BITTER  ROT" 
OF  CURRANTS 

Produced  by  the  parasitic  fungus 
(Gleosporium) .  Most  of  the  currants 
have  dropped  from  the  stem  and  the 
rest  are  rotted. 


diseases;  Figs.  109  and  110.  In  other  cases,  the  effect  upon 
the  host  is  far  less  serious.  Some  parasites  may  live  upon  a 
host  without  seriously  affecting  it.  For  example,  a  number  of 
bacteria  live  in  our  intestines;  they  may  be  called  parasitic,  since 
they  dwell  within  a  living  host;  but  instead  of  being  injurious, 


THE  MUTUAL  RELATIONS  OF  ORGANISMS  233 

some  of  them  are  beneficial  to  our  life,  and  therefore  are  not 
true  parasites,  according  to  the  definition  given  above.  Between 
these  two  extremes  are  many  intermediate  grades.  As  a  yule, 
parasitism  injures  the  host,  and  indeed,  strictly  speaking,  para- 
sitism is  a  term  that  should  only  be  used  when  one  animal  or 
plant  feeds  upon  another,  to  the  distinct  detriment  of  the  latter. 
Upon  the  Parasite. —  The  effect  of  parasitism  upon  the 
parasite  itself  is  no  less  profound  than  its  effect  upon  the  host, 
but  it  is  of  a  totally  different  nature.  The  general  effect  of 
parasitism  is  to  cause  degradation  of  the  parasite.  It  is  a 
general  law  of  living  nature  that  any  organs  which  are  not  used, 
inevitably  begin  to  degenerate.  If  an  animal  becomes  a  parasite 
upon  another,  it  shows  a  general  tendency  to  lose  many  of 
its  original  characters.  For  example,  the  tapeworm  has  become 
parasitic  in  the  intestines  of  animals.  Here  it  finds  its  food 
already  digested  by  the  digestive  juices  of  the  host;  it  has  thus 
no  need  of  a  mouth,  of  digestive  organs,  or  of  any  power  of 
motion;  and,  in  conformity  with  the  above  law  of  nature, 
having  no  need  of  these  functions,  it  has  lost  them.  The  tape- 
worm has  thus  become  degraded  to  a  very  simple  organism, 
without  digestive  organs  and  with  all  of  its  systems  of  organs 
reduced  to  the  lowest  possible  condition.  Thus,  parasites, 
depending  as  they  do  upon  their  host  for  their  nourishment, 
lose  their  power  of  independence  and  become  degraded.  This 
is  a  biological  law  of  great  significance, —  the  law  that  failure  to 
use  any  function  results  in  its  loss, —  running  through  the  whole 
scale  of  nature.  It  is  exemplified  in  the  human  race  in  numer- 
ous aspects  of  civilized  life,  where  one  class  of  people  depends 
upon  another.  In  our  highly  organized  cities  this  principle  of 
loss  of  power  as  a  result  of  disuse  is  as  well  illustrated  as  it  is 
among  animals,  since  in  the  city  individuals  are  so  mutually 
dependent  that  each  one  has  practically  lost  his  ability  to  live 
by  himself  unaided  by  others.  The  principle  of  the  loss  of 
function  by  disuse  is  one  of  the  most  fundamental  and  sig- 
nificant of  the  laws  of  nature. 


234  BIOLOGY 

NATURE'S  LIFE  CYCLE 

Construction  and  Destruction. —  From  a  general  survey  of 
the  facts  which  have  thus  been  explained,  it  will  be  seen  that 
there  is  a  grand  cycle  in  nature,  in  which  the  life  of  animals 
and  plants  is  concerned.  All  organisms  need  food,  and  the  only 
explanation  of  the  fact  that  the  food  supply  has  not  long  since 
been  exhausted  is  the  fact  that  the  same  materials  have  been 
used  over  and  over  again,  passing  from  plants  to  animals  and 
from  animals  to  plants.  The  chemical  processes  going  on 
in  the  living  world  are  of  two  types:  those  of  construction 
(synthetical),  by  which  complex  substances  are  built  out  of 
simple  ones;  and  those  of  destruction  (analytical),  by  which 
the  complex  materials  are  reduced  to  simpler  ones.  Green 
plants  growing  in  sunlight  manufacture  starch  out  of  the 
simple  ingredients  which  they  extract  from  the  soil  and  the 
air,  utilizing  sunlight  as  a  source  of  energy  for  this  purpose. 
Though  they  are  building  up  these  materials  primarily  for  their 
own  life,  they  build  more  than  they  need,  so  that  there  is  a 
large  surplus.  This  surplus  is  utilized  by  animals  and  by  the 
colorless  plants.  It  is  taken  into  their  bodies  as  food,  and  serves 
them  as  a  source  of  energy,  as  well  as  material  out  of  which 
they  can  manufacture  new  substances,  and  grow.  Eventually 
the  material  is  broken  to  pieces  in  the  animal  body  and  reduced 
once  more  to  a  simpler  condition.  In  this  way  animals  utilize 
as  food  a  part  of  the  surplus  manufactured  by  green  plants, 
consuming  the  surplus  of  proteids,  starches,  etc.  But  other 
materials  made  by  the  plants,  like  wood  and  leaves,  do  not 
so  readily  serve  as  food  for  animals.  These  materials  must 
usually  be  broken  down  into  simpler  compounds,  or  the  sub- 
stance of  which  they  are  made  would  not  get  into  a  condition 
where  it  could  again  be  utilized.  This  seems  to  be  the  special 
function  of  the  Fungi. 

The  Significance  of  the  Fungi  in  Nature. —  Special  emphasis 
must  be  given  to  the  significance  of  the  Fungi  in  these  de- 
structive processes.  In  order  that  nature's  processes  may 


THE  MUTUAL  RELATIONS  OF  ORGANISMS  235 

continue  indefinitely,  all  kinds  of  material  that  have  been 
built  up  into  organic  compounds  by  the  green  plant  must  be 
pulled  to  pieces  again  so  as  to  be  brought  back  into  the  simple 
condition  in  which  the  future  generations  of  plants  can  utilize 
them.  While  animals  use  and  break  down  much  of  the  pro- 
teids,  starches,  and  fats,  there  are  some  substances  that  ani- 
mals cannot  utilize,  and  the  Fungi  are  necessary  to  reduce 
these  substances  to  a  simpler  condition.  Bacteria  everywhere 
in  nature  are  constantly  feeding  upon  many  kinds  of  organic 
substances,  but  primarily  upon  those  that  contain  proteids  or 
other  nitrogenous  compounds.  The  yeasts  have  a  special  re- 
lation to  sugar;  most  of  the  sugars  made  by  plants,  and  not 
otherwise  used,  are  consumed  by  yeasts  in  fermentation  and  are 
thus  brought  back  to  the  original  condition  of  carbon  dioxid  and 
water.  Bacteria  and  yeasts  as  well  as  animals  thus  feed  upon  the 
same  substances.  But  there  is  other  material  of  harder  nature, 
like  wood  and  leaves,  which  does  not  serve  as  food  for  animals 
nor  to  any  great  extent  for  bacteria  or  yeasts.  The  molds, 
mushrooms,  and  tree  fungi  seem  to  be  especially  designed  by 
nature  to  attack  these  hard  materials  and  reduce  them  to  a 
condition  in  which  they  can  be  destroyed.  These  larger  fungi 
consist  of  a  mycelium  of  delicate,  branching  threads.  If  one 
of  these  plants  starts  to  grow  on  the  trunk  of  a  tree,  the  my- 
celium pushes  its  way  through  the  bark  and  in  among  the  wood 
fibers,  and  eventually  grows  through  the  whole  substance  of 
the  tree,  the  part  visible  on  the  outside  of  the  trunk  being 
only  the  spore-producing  portion  that  has  come  to  the  surface 
to  distribute  the  spores  to  other  trees.  The  mycelium,  while 
growing  within  the  wood,  produces  certain  substances  which 
soften  the  wood  and  in  time  disintegrate  it,  i.  e.,  cause  it  to 
rot.  A  tree  attacked  by  one  of  these  Fungi  in  time  becomes 
soft  and  so  changed  in  its  chemical  nature  that  it  can  be  utilized 
as  the  food  of  some  insect.  Eventually  the  trunk  of  the  tree 
is  converted  largely  into  a  soft,  pulpy  mass,  until  finally  it  is 
wholly  decomposed.  Its  carbon  and  hydrogen  unite  with 


236  BIOLOGY 

oxygen,  forming  CO2  and  H2O,  which  pass  off  into  the  air  or 
sink  into  the  soil,  while  the  other  ingredients  are  incorporated 
with  the  substances  of  the  soil  to  form  food  for  the  next  genera- 
tion of  plants. 

The  Fungi  thus  have  the  extremely  important  function  of 
bringing  back  into  a  primitive  condition  much  of  the  material 
manufactured  by  plants  which  otherwise  could  not  readily  be 
disposed  of.  When  we  consider  that  bacteria  are  nature's 
agents  for  decomposing  proteids,  that  the  yeasts  act  in  a  similar 
way  upon  carbohydrates,  and  that  the  larger  Fungi  attack  the 
great  mass  of  vegetable  material  which  is  otherwise  beyond 
the  reach  of  animal  life,  we  can  see  that  the  group  of  Fungi  is 
of  immense  significance  in  nature.  They  form  a  connecting 
link  between  the  products  of  one  generation  of  plants  and  the 
next.  Without  their  agency,  organic  material — proteids,  fats, 
starches,  leaves,  woods,  etc. —  would  accumulate,  and  in  time 
vegetation  would  cease,  because  the  earth  would  be  covered 
with  the  remains  of  past  generations,  which  would  crowd  life 
out  of  existence.  The  Fungi  thus  act  as  scavengers,  cleaning  up 
the  surface  of  the  earth  and  rendering  nature's  processes  con- 
tinuous by  ever  returning  to  the  soil  the  ingredients  upon  which 
subsequent  generations  can  feed. 

The  Food  Cycle  Complete. —  Thus,  as  the  result  of  the  action 
of  the  Fungi  and  of  animals,  all  of  the  surplus  starch  and  sugar, 
all  the  fat,  proteids,  wood,  and  cellulose,  and  indeed  all  other 
materials  which  have  been  built  up  by  the  constructive  processes 
of  plants,  are  eventually  broken  down,  and  in  the  end  reach 
a  condition  like  that  from  which  they  started.  Carbon  dioxid 
and  water  are  produced,  as  well  as  nitrates  and  certain  other 
mineral  salts.  The  carbon  dioxid,  being  a  gas,  flies  off  into  the 
air  to  join  the  store  of  this  gas  in  the  atmosphere;  the  water 
evaporates  or  sinks  into  the  soil;  and  the  nitrates  and  other 
mineral  ingredients  also  find  their  way  into  the  soil.  These 
ingredients,  again  within  reach  of  plant  life,  are  seized  by  the 
green  plants  and  built  up  into  a  new  generation  of  plants  to 


THE  MUTUAL  RELATIONS  OF  ORGANISMS  237 

make  new  starch,  sugar,  proteids,  etc.  The  ingredients  which 
feed  one  generation  of  plants  may,  after  combination  in  the 
plant  body,  nourish  a  generation  of  animals,  eventually  return- 
ing to  the  same  conditions  as  those  from  which  they  started. 
The  cycle  is  thus  complete,  and  there  need  be  no  danger  of 
exhaustion  of  the  food  supply  as  long  as  it  is  possible  for  the 
same  materials  to  be  used  over  and  over  again  by  green  plants, 
animals,  and  fungi 


CHAPTER  XII 

REPRODUCTION:    SEXUAL  AND  ASEXUAL  METHODS 
GENERAL  TYPES  OF  REPRODUCTION 

THE  process  by  which  reproduction  is  brought  about  is  always 
fundamentally  the  same.  In  spite  of  all  of  the  numerous 
modifications  of  the  method  in  different  animals  and  plants, 
they  are  all  reducible  to  some  form  of  division;  the  original 
animal  or  plant  divides  itself  into  parts,  each  of  which  is  ca- 
pable of  growing  into  an  individual  like  the  one  from  which  it 
came. 

The  numerous  varieties  of  reproduction  may  be  grouped 
together  under  two  general  types.  In  one  of  these  the  original 
organism  divides  itself  directly  into  two  or  more  parts  by 
simple  division.  In  the  other  the  division  is  always  compli- 
cated by  the  union  of  two  parts  with  each  other.  In  the  latter 
case  certain  cells  of  the  original  organisms  unite  with  each 
other,  and  the  union  is  followed  by  a  rapid  division  of  the  cells. 
The  two  types  of  reproduction  are,  therefore,  (1)  Division  un- 
accompanied by  cell  union  and  (2)  Division  accompanied  by 
cell  union.  The  type  of  division  in  which  cell  union  is  found 
is  often  spoken  of  as  sexual  reproduction,  and  the  uniting  cells 
are  the  sex  cells;  the  type  in  which  the  division  is  not  accom- 
panied by  cell  union  is  called  asexual  reproduction. 

REPRODUCTION  IN  UNICELLULAR  ORGANISMS 
Simple  Division. —  All  of  the  single-celled  animals  multiply 
by  the  process  of  simple  division;  Figs.  19,  23.  A  careful 
study  of  the  internal  changes  that  are  going  on  in  the  celk 
during  this  reproduction  shows  that  they  are  essentially 
identical  with  those  described  on  pages  85-89.  In  other  words> 
there  is  a  division  of  the  chromatin  material  in  the  nucleus, 
followed  by  the  formation  of  two  nuclei,  which  again  is  fol- 
lowed by  the  division  of  the  cell  into  two  parts.  After  having 

238 


SEXUAL  AND  ASEXUAL  REPRODUCTION  239 

thus  divided  and  separated  from  each  other,  each  of  the  indi- 
viduals grows  until  it  is  ready  to  divide,  and  so  the  process 
goes  on  repeating  itself.  In  most  unicellular  plants,  the  method 
of  reproduction  is  essentially  the  same.  Figure  30,  for  example, 
shows  the  reproduction  in  Pleurococcus,  and  Figure  33  in  ordi- 
nary bacteria.  These  latter  plants  are  so  small  that  we  cannot 
determine  the  internal  changes  that  are  going  on,  but  can  only 
see  that  the  individuals  elongate  and  then  divide  in  the  middle, 
into  two  parts.  Recent  study,  however,  seems  to  suggest  that 
the  changes  are  essentially  like  those  occurring  in  the  Amoeba, 
and  at  all  events  the  process  of  reproduction  is  nothing  more 
than  the  process  of  division. 

The  reproduction  of  yeast  by  budding  (gemmation)  is  only 
a  modification  of  division;  Fig.  32.  The  internal  changes  are 
essentially  like  those  in  the  reproduction  of  the  Amoeba  or 
Paramecium;  the  first  step  is  the  division  of  the  nucleus  into 
two,  one  of  which  passes  out  of  the  original  cell  into  the  bud, 
while  the  other  remains  in  the  original  cell.  Thus,  when  the 
two  cells  separate,  each  has  a  nucleus  that  has  come  from 
the  original  nucleus,  and,  while  the  details  of  the  process  are 
somewhat  different,  it  is  as  truly  a  cell  division  as  in  the  other 
examples.  Nearly  all  of  the  unicellular  animals  and  plants 
show  one  of  these  two  methods  of  reproduction;  see  Fig.  111. 

Reproduction  by  Spores. —  When  the  organism  breaks  up 
into  many  parts,  they  are  called  spores.  Examples  of  this 
we  have  already  noticed  among  the  unicellular  organisms.  In 
the  yeast  (Fig.  32  s),  spores  are  formed  within  the  yeast  cells 
under  some  conditions;  and  Figure  25,  which  shows  the  life 
history  of  the  malarial  organism,  indicates  that  one  part  of 
its  history,  namely,  the  cycle  in  the  human  blood,  is  an  illus- 
tration of  spore  formation.  In  the  malarial  Plasmodium  the 
spore  formation  which  occurs  in  the  human  blood  alternates 
with  a  second  type  of  spore  formation  in  the  body  of  the  mos- 
quito. This  last  process  is,  however,  associated  with  celi 
union,  as  shown  in  Figure  25  j.  Among  unicellular  animals 


240 


BIOLOGY 


spore  formation  is  unusual,  except  in  cases  where  it  alternates 
with  a  cell  union,  as  in  Plasmodium.    Among  bacteria  there  is 

a  spore  formation  of  a  peculiar  kind. 
Here,  as  shown  in  Figure  33  E,  each 
bacterium  produces  a  single  spore  only, 
instead  of  several,  and  the  spore  forma- 
tion is  really  not  a  form  of  multiplica- 
tion. The  cells  formed  are,  however, 
called  spores,  although  their  function 
seems  to  be  to  resist  adverse  condi- 
tions rather  than  to  reproduce  the  or- 
ganism. They  have  resisting  walls  and 
are  capable  of  developing  into  new  in- 
dividuals, thus  agreeing  with  other 
spores  except  in  the  fact  that  one  only 
is  produced  in  a  cell. 

Reproduction  by  Cell  Union  among 
Unicellular  Organisms. —  The  process 
of  cell  division  among  single-celled  or- 
ganisms may  continue  for  a  long  time, 
producing  an  indefinite  series  of  off- 
spring. Whether  in  any  case  this  kind  of  division  can  really 
go  on  indefinitely  we  do  not  positively  know.  There  are  some 
organisms  like  yeast  and  bacteria,  in  which  we  have  reason 
for  suspecting  that  cell  division  may  go  on  indefinitely  if  proper 
conditions  can  be  maintained,  and  in  which,  up  to  the  present, 
no  trace  of  any  other  kind  of  reproduction  has  been  found. 
It  is  believed  by  some  that  even  animals  like  Paramedum, 
which  conjugate  occasionally,  may,  if  proper  conditions  be 
maintained,  go  on  dividing  indefinitely.  Whether  this  is  true 
or  not,  it  is  certain  that  under  ordinary  conditions  cell  division 
in  time  becomes  slower,  and  in  Paramedum  it  has  a  tendency 
to  come  to  an  end,  unless  it  is  reinvigorated  in  some  way. 
In  nature  such  an  invigoration  is  brought  about  by  a  fusion 
of  cells  with  each  other  as  already  described;  see  Fig.  23,  page  64. 


FIG.  111. — GONIUM.  AN 
ORGANISM  FORMED  OP 
SIXTEEN  CELLS  UNITED 

BY  JELLY 

A,  view  from  the  side;  B, 
view  from  above  and  showing 
the  method  of  reproduction  by 
division  of  each  cell  into  sixteen 
parts,  which  separate  to  form 
new  colonies. 


SEXUAL  AND  ASEXUAL  REPRODUCTION  241 

It  is  probable  that  in  most  other  unicellular  organisms  a 
similar  cell  union  occurs  under  some  conditions.  As  already 
described,  it  occurs  in  the  malarial  organism  in  the  cycle  that 
takes  place  in  the  mosquito;  Fig.  25  j.  The  cell  union  that 
takes  place  is  a  true  sex  union,  since  there  is  a  clear  dis- 
tinction of  male  and  female  cells.  While  such  a  union  of  cells 
has  by  no  means  been  found  in  ail  unicellular  organisms,  it 
has  been  found  in  many,  and  we  know  that  it  is  quite  widely 
distributed.  The  studies  of  recent  years  particularly  have 
shown  one  large  group  of  unicellular  organisms  called  the 
Sporozoa,  which  live  as  parasites  on  various  hosts,  and  show 
a  cell  union  resembling  that  of  malaria.  Another  example  of 
this  will  be  given  here  in  illustration  of  the  phenomenon  of  cell 
union  among  single-celled  animals. 

Monocystis. —  In  the  earthworm  may  be  found  living  a 
unicellular  parasite  known  as  Monocystis.  This  animal  (see 
Fig.  112  A)  is  a  single  elongated  t  cell  possessing  a  nucleus,  but 
with  no  other  visible  organs.  The  animal  has  no  locomotor 
organs,  although  it  does  have  a  slight  power  of  motion.  Its 
method  of  reproduction  involves  a  cycle,  in  which  a  cell  union 
alternates  with  a  formation  of  spores  without  cell  union,  but 
in  a  complicated  manner.  When  ready  to  multiply,  two  indi- 
viduals fuse  with  each  other  and  become  surrounded  by  a 
covering  or  cyst;  Fig.  B.  Inside  of  this  cyst  both  of  the  indi- 
viduals divide.  First  the  nucleus  divides  into  many  parts 
(see  Fig.  C),  and  later  the  rest  of  the  protoplasm  divides  and 
collects  around  the  pieces  of  the  divided  nuclei,  thus  making 
many  small  cells.  Now  the  new  cells  from  one  of  these  indi- 
viduals unite  in  pairs  with  the  cells  from  the  other.  This  step 
occurs  within  the  cyst,  but  is  shown  separated  from  it  in  Figure 
Dj  a,  by  and  c,  and  it  constitutes  the  cell  union  proper.  When 
the  cells  fuse  together  their  nuclei  unite,  forming  a  single  nu- 
cleus, c,  called  the  fusion  nucleus,  which  divides  into  eight 
parts,  at  e,  after  which  the  whole  cell  divides  into  eight  elon- 
gated cells  (see  /)  known  as  sporozoites.  Meantime  a  hard 


242 


BIOLOGY 


shell  is  produced  around  the  eight  sporozoites  and  the  whole 
cluster  of  eight  is  called  a  sporoblast.  All  of  this  has  occurred 
within  the  original  cyst,  which  has  by  this  time  become  filled 
with  a  large  number  of  these  sporoblasts,  each  with  its  eight 


FIG.  112. — MONOCYSTIS,  SHOWING  METHOD  OP  REPRODUCTION 

A,  the  full-grown  animal;  B,  two  individuals  enclosed  in  a  cyst;  C,  the  division  of  the 
nucleus  into  a  number  of  parts,  the  protoplasm  not  yet  divided;  D,  successive  stages  of  the 
fusion  of  thfe  cells  which  result  from  division  of  the  two  animals  in  C;  F,  the  cyst  containing 
numerous  sporoblasts;  G,  shows  the  sporoblast  breaking  open  to  allow  the  spores  to  emerge, 
which  develop  into  adult  animals.  The  stages  represented  in  G  occur  only  when  the  animal 
•caches  another  earthworm. 

sporozoites  within;  see  Fig.  F.  Eventually  the  cyst  breaks 
open,  allowing  the  contents  to  escape.  Later  these  sporcblasts 
themselves  break  open  and  the  individual  sporozoites  come  but 
ready  to  grow  into  new  animals  like  the  original  Monocystis; 
Fig.  G.  These  latter  stages  do  not  occur  unless  the  sporozoites 
find  their  way  into  another  earthworm,  where  they  live  as 
parasites  until  ready  to  multiply  again.  The  sporozoites  are 


SEXUAL  AND  ASEXUAL  REPRODUCTION  243 

evidently  spores,  but  they  arise  from  the  division  of  a  mass 
resulting  from  the  fusion  of  two  reproductive  cells;  and  to 
distinguish  them  from  other  spores  they  are  called  sporozoites. 
By  comparing  this  history  with  that  of  the  malarian  Plasmo- 
dium,  it  will  be  evident  that  the  spores  of  the  latter,  which  are 
formed  in  the  body  of  the  mosquito,  must  be  sporozoites,  since, 
like  those  just  described,  they  arise  from  the  breaking  up  of 
the  mass  of  the  two  cells  which  have  united  by  cell  union. 
Monocystis  as  here  described  shows  no  spores  which  correspond 
to  those  that  appear  in  the  human  blood;  Fig.  25  g  and  h. 

REPRODUCTION  IN  MULTICELLULAR  ORGANISMS 

Multicellular  organisms  have  the  same  two  general  types  of 
reproduction  as  found  in  the  unicellular;  namely,  simple  divi- 
sion, and  division  accompanied  by  cell  union. 

DIVISION  WITHOUT  CELL  UNION 

Multiplication  by  Simple  Division. —  Simple  division  among 
multicellular  organisms  is  more  common  among  plants  than 
among  animals;  and  excellent  examples  of  it  are  familiar  to  all. 
Many  of  the  lower  plants,  like  liverworts,  multiply  by  the  for- 
mation of  buds  called  gemmae,  which  break  away  from  the 
original,  and  form  new  plants.  Even  among  the  higher  plants 
the  same  general  method  is  found.  If  one  of  the  branches  of 
a  weeping-willow  tree  is  broken  off  and  stuck  into  moist  ground, 
it  will  take  root  and  grow  into  a  new  tree.  Indeed,  we  can 
cut  the  branches  of  a  willow  into  practically  as  many  pieces 
as  we  wish,  and  find  each  one  is  capable  of  taking  root  and 
growing  into  a  new  tree.  The  same  thing  is  true  of  most  ordi- 
nary plants,  for,  with  a  few  exceptions,  trees  and  smaller  plants 
may  be  reproduced  indefinitely  by  breaking  off  their  branches 
and  putting  them  into  the  proper  conditions  for  taking  root. 
While  a  few  plants  fail  to  show  this  power,  it  is  a  character 
found  very  commonly  in  the  vegetable  kingdom.  Many  plants 
normally  multiply  upon  a  similar  principle.  The  strawberry 


244  BIOLOGY 

plant,  for  example,  sends  out  branches  which  grow  for  some 
distance,  and  then  their  tips  strike  root  into  the  ground  and 
a  new  plant  springs  up,  united  with  the  old  one  at  first  by  a 
connecting  branch;  Fig.  113. 

Among  animals  this  method  of  reproduction  is  not  so  common 
as  in  plants  and  is  confined  to  the  lower  species.  One  example 
has  been  already  described  in  Hydra;  see  page  146. 

Reproduction  by  division  is  evidently  closely  related  to  the 
power  of  replacing  lost  parts.  Hydra  may  be  divided  into  many 


FIG.  113. —  REPRODUCTION  IN  A  STRAWBERRY  PLANT  BY  DIVISION 

pieces,  each  capable  of  producing  all  of  its  lacking  parts;  but 
this  power  is  retained  in  diminishing  degree  as  we  go  from 
lower  to  higher  animals.  The  earthworm  does  not  ordinarily 
multiply  by  simple  division,  but  if  it  is  cut  into  two  pieces  by 
accident,  each  will  develop  the  lost  parts  and  two  animals 
will  result.  In  some  worms,  related  to  the  earthworm,  this 
method  of  multiplication  by  division,  each  piece  developing  all 
of  the  lost  parts,  is  a  normal  method  of  reproduction;  Fig.  114. 
Reproduction  by  Spores. —  Reproduction  by  means  of  spores 
is  also  found  among  the  multicellular  organisms,  especially 
among  the  multicellular  plants.  A  few  illustrations  of  it  are 
the  following. — 


SEXUAL  AND  ASEXUAL  REPRODUCTION 


245 


Examples  of  spore  formation  in  molds  have  already  been 
described  (page  97),  two  methods  having  been  mentioned. 
In  Mucor  (Fig.  42  E)  the  spores  are  pro- 
duced within  a  sac  called  a  sporangium, 
while  in  Penidllium  (Fig.  42  A)  they  are 
only  the  ends  of  branches,  growing  in  the 
air.  The  latter  are  called  conidia  to  dis- 
tinguish them  from  spores  formed  in  spo- 
rangia. The  nature  and  function  of  spores 
and  conidia  are  the  same. 

Another  well-known  illustration  of  the 
same  is  the  common  puffball.    This  is  a 
colorless  plant,  growing  from  a  mycelium 
which   lies  chiefly  below  the  surface  of 
the  ground.    At  certain  seasons  of  the 
year    there    arise   from    the    mycelium, 
rounded  knobs  which 
rapidly  increase  in  size. 
They  may  grow  as 
large  as  a  walnut  or 
an  orange,  and  in  some 
species  they  reach  a 
diameter  of  a  foot,  or 
even  two  feet ;  see  Fig. 
115.  Within  this  great 
mass  the  contents  di- 
vide into  millions  of  spores,  and  after  they 
have  been  properly  matured    an   opening 
appears  at  the  top  and  the  spores  emerge  in 
the  form  of  a  fine  dust.    The  slightest  touch 
upon  the  puffball  will  throw  masses  of  dust 
into  the  air,  from  which  arises  the  name  puff- 
ball.     This  dust  consists  of  millions  of  minute  spores,  each  of 
which  can  become  a  new  plant. 

This  power  of  producing  spores  is  widely  distributed  among 


FlG.  114. TWO  SEG- 
MENTED WORMS, 
WHICH  MULTIPLY  BY 
ASEXUAL  METHODS 

A,  Autolytus,  multiplying 
by  division; B,  Syllis,  multi- 
plying by  budding,  the  buds 
growing  from  the  side  and 
breaking  away  to  form  new 
individuals. 


FlG.  115. A  PUFF- 
BALL  SHOWING 
THE  SPORES  PRO- 
TRUDING FROM 
THE  OPENING 


246  BIOLOGY 

plants,  occurring  in  the  lower  as  well  as  in  the  higher.  Even 
in  the  flowering  plants  the  pollen  of  a  flower  is  really  a  mass 
of  spores,  although  their  relation  to  the  growth  of  the  plant  is 
different  from  that  of  the  spores  to  the  puffball,  since  they  do 
not  grow  immediately  into  a  plant  like  the  one  that  produces 
them. 

Among  the  multicellular  animals,  the  production  of  spores 
is  not  found.  There  is,  however,  in  a  few  animals  a  method 
of  reproduction,  called  parthenogenesis,  which  in  some  respects 
resembles  spore  formation.  The  essential  differences  between 
reproduction  by  spores  and  that  by  eggs  is  that  a  spore 
grows  into  a  new  organism  without  being  united  with  a 
sperm,  i.  e.,  no  fertilization  is  required  (see  page  267),  while 
an  egg  must  combine  with  a  sperm  in  order  to  be  capable  of 
growing  into  a  new  organism.  Some  organisms,  however, 
produce  eggs  that  can  grow  without  fertilization.  Among  the 
best-known  examples  of  this  is  the  honey  bee.  The  female 
bee  produces  true  eggs,  some  of  which  unite  with  sperms,  while 
others  develop  without  such  union.  The  individuals  produced 
from  the  unfertilized  and  from  the  fertilized  egg  are  different, 
the  fertilized  eggs  producing  worker  bees  or  females,  and  the 
unfertilized  eggs  producing  males  (drones).  So  far  as  can  foe 
seen  the  eggs  are  alike,  the  only  difference  between  the  eggs 
that  produce  workers  and  those  that  produce  males  being  that 
one  is  fertilized  and  the  other  not.  This  phenomenon  of  the 
development  of  eggs  without  fertilization  is  called  partheno- 
genesis (Gr.  parthenos  =  virgin  +  genesis  =  creation) .  It  re- 
sembles reproduction  by  spores  only  in  the  fact  that  it  consists 
of  a  single  cell  developing  into  an  adult  without  the  neces- 
sity of  union  with  a  sperm;  but  the  reproductive  bodies  are 
identical  with  eggs,  and  it  is  usually  described  as  reproduction 
by  eggs  which  do  not  require  fertilization. 

Parthenogenesis  occurs  in  a  variety  of  animals  with  vari- 
ous complications.  Where  it  occurs  it  is  most  common  to 
have  such  a  parthenogenetic  reproduction  alternate,  with  more 


SEXUAL  AND  ASEXUAL  REPRODUCTION 


247 


7 


or  less  frequency,  with  sexual  reproduction.  In  the  microscopic 
Animal  Hydatina,  for  example  (Fig.  116),  found  in  fresh  water, 
the  eggs  commonly  produced,  called  summer  eggs,  develop 
without  fertilization  into  new  females,  which  rapidly  mature 
and  produce  more  similar  eggs  that  develop  in  the  same  way. 
This  may  go  on  for  a  long  time,  under  proper  conditions  for 
hundreds  of  generations,  without  any 
males  making  their  appearance.  Even- 
tually, however,  under  conditions  not 
yet  understood,  males  make  their  ap- 
pearance and  the  females  produce  eggs 
of  a  different  kind,  called  winter  eggs, 
which  are  incapable  of  developing  with- 
out being  combined  with  sperms  by  the 
sexual  process.  Here,  then,  partheno- 
genesis seems  to  be  the  normal  method 
of  reproduction,  sexual  reproduction 
alternating  with  it  at  unknown  and  un- 
certain intervals.  The  reasons  for  this 
alternation,  and  the  conditions  that  de- 
termine the  one  or  the  other  method, 
are  not  yet  understood. 


FIG.  116.— HYDATINA.  A 

MICROSCOPIC  ORGANISM 
THAT  MULTIPLIES  BY 
PARTHENOGENESIS 


ex,  excretory  system; 
gl,  gland; 
m,  mouth; 
st,  stomach. 


MULTIPLICATION  BY  CELL  UNION 

Conjugation. —  In  all  animals  above 
che  unicellular  forms,  and  in  most 
plants,  cell  union  is  found  as  a  factor 
in  reproduction.  Among  a  few  plants 

of  the  lower  orders  the  cells  which  unite  are  alike.  In 
Mucor,  for  example,  besides  the  spore  formation  mentioned 
on  page  97,  a  union  of  cells  sometimes  takes  place;  Fig.  117. 
As  shown  in  Figure  A,  special  lateral  threads  grow  out  from 
the  ordinary  mycelium  of  the  mold,  and  these  come  in  contact 
with  each  other  at  their  tips.  After  they  touch  each  other 
single  cells  are  divided  off  from  each,  B,  which  fuse  with 


248 


BIOLOGY 


each  other,  as  shown  at  C.  This  fused  mass  is  called  a  zygo- 
spore  (Gr.  zygon  =  yoke  +  spora),  z.  It  enlarges,  becomes 
covered  with  a  hard  case,  D,  and  breaks  away  from  the  plant 
that  produced  it.  It  may  then  remain  dormant  for  a  long  time, 
but  eventually  it  sprouts,  E,  and  grows  into  a  new  plant. 
In  this  case  the  two  cells  that  unite  are,  so  far  as  the  micro- 
scope discloses,  alike,  and  the  plants  that  produce  them  appear 
identical.  But  careful  study  has  proved  that  there  is  a  differ- 
ence in  the  uniting  plants,  shown  not  in  their  shape,  but  in 
their  uniting  powers.  It  has  been  found  that  there  are  two 


D  E 

FIG.  117. —  CONJUGATION  OF  MUCOR 

Successive  stages  being  shown  in  A  toE;  x  and  y  are  threads  from  different  plants,  which 
unite  by  conjugation;  z,  the  zygospore;  at  E  the  zygospore  has  sprouted  to  form  a  new 
plant. 

types  of  Mucor,  differing  only  in  their  power  of  uniting  with 
each  other.  For  example,  in  Figure  A,  the  two  different  my- 
celium threads  are  marked  x  and  y.  It  is  found  that  while 
outgrowths  of  x  can  unite  with  outgrowths  of  ?/,  they  can 
never  unite  with  other  outgrowths  of  the  mycelium  x.  There 
are  thus  two  different  types  of  plants,  each  capable  of  uniting 
with  the  other,  but  not  capable  of  uniting  with  outgrowths  from 
itself.  This  reminds  us  of  sex  union,  where  an  egg  will  unite 
with  a  sperm  but  not  with  another  egg.  It  cannot  be  called 
true  sex,  however,  since  there  are  no  distinguishable  differences 


SEXUAL  AND  ASEXUAL  REPRODUCTION 


24S 


FIG.  118.— EGGS.   A, 
OF  'AN  ANIMAL;  B, 

<fF  A  PLANT 


between  the  uniting  bodies.  It  is  thought  to  be  a  first  step 
toward  the  true  sex  which  is  developed  in  higher  plants.  Since 
the  uniting  bodies  in  Mucor  are,  so  far  as  can  be  seen,  alike, 
the  union  is  called  conjugation. 

Among  multicellular  animals  conjugation 
is  unknown,  true  sex  union  being  always 
found  instead. 

Fertilization  or  Sex  Union. —  The  eggs 
of  all  organisms  consist  of  single  cells  which 
have  prominent  nuclei;  Fig.  118.  Eggs  are 
usually  rounded  in  shape,  although  they 
may  vary.  In  size  they  range  all  the  way 
from  eggs  that  are  too 
small  to  be  seen  with- 
out the  microscope,  up 
to  the  size  of  the  ostrich 
egg.  The  size  of  the  egg 
is  by  no  means  propor- 
tional to  the  size  of  the  animal  that  pro- 
duces it.  The  human  egg,  for  example,  is 
microscopic,  and  the  egg  of  the  hen  is  gigan- 
tic in  comparison.  In  large  eggs,  like  those 
of  the  hen  or  the  ostrich,  the  bulk  of  the 
egg  is  made  of  food  material,  sometimes 
called  yolk,  or  deutoplasm  (Gr.  de^teros  =  sec- 
ond +  plasma  =  substance),  deposited  within 
the  eggshell  for  the  nourishment  of  the 
young  which  is  to  be  developed.  The  egg 
has  a  thin  cell  wall  which  is  known  as  the 
vitelline  membrane. 

The  eggs  of   animals    are    produced    in 
organs  called  the  ovaries;  Fig.  119.     They 
are  situated  in  different  parts  of  the  body  in  different  animals, 
and  their  sole  function  is  to  produce  eggs,  which  are  then  car- 
ried to  the  exterior  through  ducts  called  the  oviducts.    As  can 


FIG.    119.  —  DIA- 
GRAMMATIC  SEC  - 

TION  OF  THE 
OVARY  OF  AN 
ANIMAL 


cflls 


Showing  the  origin  of 
from  the  ordinary 
,  ov  ova. 


250 


BIOLOGY 


be  seen  from  Figure  119,  the  egg  is  really  a  single  cell,  like  the 
other  cells  of  the  body  in  structure,  though  larger  in  size.  As 
the  egg  passes  along  the  oviduct  it  is  not  infrequently  sur- 
rounded with  a  mass  of  yolk  and  a  shell;  neither  the  yolk  nor 
the  shell  is  an  essential  part  of  the  egg,  the  yolk  being  a  food 
material  for  the  nourishment  of  the  embryo,  and  the  shell  be- 
ing a  covering  to  protect  the  egg  after  it  has  left  the  body. 

Plants  also  produce  eggs  similar  in  structure  to  those  of 
animals  (Fig.  118  B),  though  the  organs  that  produce  them 
are  not  called  ovaries.* 

Sperms. —  Sperms  are  extremely  minute  cells  which  must 
unite  with  the  eggs  in  order  that  the  latter  may  be  capable  of 

further  development.  Sperms  are  by 
no  means  uniform  in  shape.  As  a 
rule,  each  consists  of  a  minute  head 
and  a  motile  tail,  whose  lashing  move- 
ments propel  the  sperm  through 
liquids  until  the  sperm  is  brought  in 
contact  with  the  egg.  Figure  120 
shows  the  sperms  of  a  number  of  ani- 
mals and  plants.  There  is  great  vari- 
ety among  them,  and,  while  some  of 
them  are  provided  with  tails,  others 
are  not,  and,  although  usually  motile, 
the  sperms  of  some  animals  are  sta- 
tionary. The  sperms  of  animals  are 
produced  in  special  glands  called  sper- 
maries  or  testes.  In  the  frog  and 
earthworm  the  position  of  these  sperm 
glands  is  shown  in  Figure  80.  The  sperms  are  passed  from 
the  spermary  into  ducts,  commonly  known  as  the  vasa  defer- 
entia,  which  carry  them  to  the  exterior.  These  ducts  may  be 

*  It  will  be  noticed  that  the  ovary  of  an  animal  is  quite  different  from 
the  ovary  of  a  flower,  since  the  latter  does  not  produce  eggs  nor  have 
oviducts;  see  page  273. 


A  B 


FIG.  120. —  VARIOUS  FORMS 

OF   SPERMS 

A,  B,  C,  D,  and  E,  sperms  of 
animals;  F,  of  a  fern;  G,  of  a  liver- 


wort. 


(Various  authors.) 


SEXUAL  AND  ASEXUAL  REPRODUCTION  251 

very  short  or  they  may  be  long  and  coiled.  Sperms  are  much 
smaller  than  eggs,  the  sperm  being  always  microscopic.  Plants 
also  produce  sperms  (Fig.  120  G),  though  they  do  not  come 
from  spermaries  or  special  sperm  ducts;  see  page  271. 

Males,  Females,  and  Hermaphrodites. —  When  reproduction 
in  animals  or  plants  is  brought  about  by  eggs  and  sperms, 
the  process  is  spoken  of  as  sexual  reproduction  and  the  uniting 
bodies,  the  eggs  and  sperms,  are  sex  bodies.  The  glands  that 
produce  them  are  the  sexual  glands,  or  gonads,  and  the  ducts 
that  conduct  the  bodies  to  the  exterior  are  the  sexual  ducts. 
Among  animals,  it  is  most  common  to  have  one  individual 
produce  either  spermaries  or  ovaries,  but  not  both,  and  the 
individuals  are  then  spoken  of  as  males  and  females.* 

In  some  animals,  however,  as  already  seen  in  the  earthworm, 
the  same  individuals  may  produce  both  spermaries  and  ovaries. 
Such  individuals  are  spoken  of  as  hermaphrodites.  Among 
animals  hermaphrodites  are  found  chiefly  among  the  lower 
orders,  very  few  being  found  among  the  higher.  Among  plants, 
however,  both  hermaphroditic  and  separate  sexed  conditions 
are  common;  hermaphroditic  plants  are  called  monoecious 
(Gr.  monos  =  one  +  oikos  =  house),  and  the  separate  sexed 
plants  dioecious  (Gr.  di-  =  twice  -f-  oikos).  In  the  higher 
flowering  plants  the  relation  of  the  sexes  is  peculiar,  and  com- 
plicated by  what  is  called  alternation  of  generations,  to  be 
described  later. 

THE  UNION  OF  THE  SEX  BODIES  OR  FERTILIZATION 

The  union  of  the  egg  and  the  sperm  is  called  fertilization, 
and  the  moment  when  the  egg  and  the  sperm  unite  is  the 
beginning  of  the  life  of  the  new  individual.  This  process  of 
union  of  the  sex  bodies  is  peculiar  and  of  extreme  significance. 
In  the  description  which  follows,  the  successive  changes  which 
occur  are  described  without  reference  to  any  particular  spe- 

*The  sign  $  is  used  to  denote  the  male  sex,  9  to  denote  the  female  sex, 
and  $  to  denote  hermaphrodites. 


252  BIOLOGY 

cies.  Essentially  the  same  series  of  events  occurs  in  all  animals 
where  a  fertilization  takes  place,  although  the  order  of  events 
is  not  always  the  same.  In  a  previous  chapter  we  have  seen 
that  in  all  animals,  when  the  chromatin  of  the  nucleus  breaks 
into  chromosomes  before  division,  the  number  of  chromosomes 
is  always  the  same  in  all  cells  of  the  species.  In  order  to 
illustrate  the  process  of  the  origin  and  union  of  the  sex  cells, 
we  will  describe  the  process  in  an  animal  that  has  four 
chromosomes,  meaning  by  this  that  all  of  the  cells  of  the  ani- 
mal (except  the  germinal  cells  to  be  described)  contain  four 
chromosomes  at  the  time  when  cell  division  takes  place. 

Origin  of  the  Egg  (Oogenesis). —  The  egg  is  simply  one  of 
the  ordinary  cells  of  the  ovary.  During  the  early  life  of  the 
animal,  the  cells  in  the  ovary  increase  by  the  ordinary  process 
of  cell  division,  with  nothing  especial  to  distinguish  it  from 
the  cell  division  of  the  other  cells.  In  all  cases,  the  cells  are 
about  the  ordinary  size  and  all  contain  the  normal  number  of 
four  chromosomes.  This  process  continues  indefinitely  during 
the  early  life  of  the  animal,  until  it  is  ready  to  produce  eggs. 
When  this  time  comes,  some  of  the  cells  of  the  ovary  begin 
to  increase  greatly  in  size,  and  become  in  a  short  time  very 
much  larger  than  the  ordinary  cells,  not  only  than  the  cells 
of  the  body  generally,  but  much  larger  than  all  of  the  other  cells 
in  the  ovary.  This  increase  in  size  is  due  largely  to  deposition 
in  the  egg  of  food  material  which  is  to  serve  as  nourishment 
for  the  young  that  is  subsequently  to  develop  from  the  egg. 
At  the  time  the  egg  increases  in  size,  a  peculiar  change  takes 
place  in  the  chromosomes  within  the  nucleus.  By  a  series 
of  divisions,  this  chromatin  divides  into  a  number  of  chromo- 
somes which  is  always  double  that  found  in  the  ordinary  cells 
of  the  animal.  In  our  illustration,  instead  of  four  of  these  chro- 
mosomes, there  are  eight.  These  chromosomes  always  assume 
at  this  stage  the  arrangement  in  groups  of  fours,  such  as  is 
shown  in  Figure  121  A.  There  is  thus  produced  a  large  pri- 
mary egg  (Gr.  don  =  egg-fci/tos),  called  an  oo'cyte,  containing 


SEXUAL  AND  ASEXUAL  REPRODUCTION 


253 


an  immense  amount  of  food 
number  of  chromosomes 
that  are  found  in  the  ordi- 
nary cells  of  the  animal. 
This  doubling  of  the  chro- 
mosomes is  the  last  step  in 
the  formation  of  the  oocyte. 
Maturation  of  the  Egg. — 
At  the  stage  shown  in  Figure 
121  A,  the  egg  is  not  yet 
mature,  i.  e.,  is  not  yet  ready 
to  unite  with  the  sperm;  it 
must  first  pass  through  a 
further  series  of  changes 
spoken  of  as  the  maturation 
of  the  egg.  The  nucleus  ap- 
proaches the  edge  of  the 
egg  and  divides  into  two 
parts,  one  very  large  and 
one  very  small,  each  retain- 
ing four  of  the  chromosomes 
present  in  the  original  nu- 
cleus; Fig.  121 B-D.  It  will 
be  noticed  that  in  this  divi- 
sion the  chromosomes  do 
not  split*  as  they  do  in  or- 
dinary cell  division  (see 
page  87),  but  that  each 
of  the  two  nuclei  formed 
contains  half  of  the  original 
eight.  One  of  these  nuclei 
is  pushed  out  of  the  egg 
as  a  small  protrusion 
shown  at  D;  the  other  one 
a  short  period  of  rest  the 


yolk  in  it,  and  with  double  the 


J 

FIG.   121. —  DIAGRAM    SHOWING  THE 
MATURATION  AND  FERTILIZATION  OF 

AN   EGG 

Stages  A  to  G  represent  maturation;  H  and 
I  the  fertilization;  J,  the  egg  after  it  has  di- 
vided; /,  female  pronucleus;  ra,  male  pronu- 
cleus;  p,  polar  bodies;  sp,  a  typical  flagellate 
sperm  more  highly  magnified. 

remains  within   the  egg.     After 
process  of  division  is   repeated, 


254  BIOLOGY 

the  two  nuclei  once  more  dividing  into  two  parts  without  any 
splitting  of  the  chromosomes,  each  of  the  four  nuclei  thus  con- 
taining two  of  the  original  chromosomes.  Half  of  the  nucleus 
still  within  the  egg  is  extruded,  while  the  other  half  remains 
within;  Fig.  121  E,  F.  The  nuclei  which  are  thus  extruded  from 
the  egg  are  called  polar  cells,  p,  and  have  no  further  function, 
since  they  have  nothing  to  do  with  the  individual  which  is  to 
arise  from  the  egg.  They  are  rejected  products  and  soon  dis- 
appear. After  the  nuclei  have  divided  the  second  time,  the 
nucleus  remaining  within  the  egg,  with  its  two  chromosomes, 
once  more  passes  toward  the  center  of  the  egg  and  is  called 
the  female  pronucleus;  Fig.  G,  f.  The  egg  is  now  ready  to 
unite  with  the  sperm.  The  egg,  in  other  words,  has  become 
mature,  this  process  of  the  extrusion  of  the  three  small  nuclei 
being  the  essential  feature  of  the  process  of  maturation. 

The  Origin  of  the  Sperm  (Spermatogenesis) . —  The  origin  of 
the  sperm  is  essentially  similar  to  that  of  an  egg,  differing, 
however,  in  one  rather  important  point.  As  in  the  ovary, 
the  ordinary  Cells  in  the  sperm  glands,  during  the  early  life 
of  the  animal,  continue  their  growth  and  division  by  the  process 
of  simple  cell  division,  with  the  normal  method  of  the  division 
of  the  chromosomes.  When,  however,  the  sperms  are  about 
to  be  formed,  the  cells  of  the  spermary  undergo  a  change  simi- 
lar to  that  described  in  the  formation  of  the  egg,  except  that 
they  do  not  materially  increase  in  size.  In  each  of  these  cells, 
called  a  spermocyte  (Gr.  sperma  =  germ  +  cytos),  the  number 
of  chromosomes  doubles  itself,  producing  a  number  identical 
with  that  found  in  the  oocyte;  Fig.  122  7.  The  chief  difference 
between  this  spermocyte  and  the  oocyte  at  the  corresponding 
stage  is  that,  whereas  the  egg  has  greatly  increased  in  size  by 
the  deposition  of  the  food,  the  cell  which  is  to  form  the  sperm 
does  not  increase  in  size. 

The  next  step  in  the  development  of  the  sperm  is  the  divi- 
sion of  this  cell  into  four  parts.  This  step  corresponds  clearly 
with  the  division  of  the  egg  cell  into  four  parts,  as  shown  in 


SEXUAL  AND  ASEXUAL  REPRODUCTION  255 

Figure  122  //  to  ///.  In  this  case,  however,  the  division  does 
not  produce  one  large  and  three  small  cells,  but  four  cells  of 
equal  size,  each  one  of  which  receives  two  of  the  chromosomes. 
It  is  evident,  therefore,  that  one  of  these  cells  is  equivalent  to 


FIG.   122. —  DIAGRAM  SHOWING  A  COMPARISON  BETWEEN   THE  MATURA- 
TION  OF  AN   EGG,  B,  AND   THE   FORMATION   OF  THE   SPERMS,   A 
Stages  /  to  IV  in  series  A  and  B  correspond  with  each  other. 

one  of  the  cells  developing  in  the  maturation  of  the  egg,  at 
least  so  far  as  concerns  its  nuclear  matter  and  its  chromosomes, 
differing,  however,  in  the  amount  of  cell  substance  that  may 
be  present.  In  the  further  development  we  find  another  point 
of  difference  in  the  fact  that  each  one  of  these  four  cells  de- 
velops into  a  perfect,  functional  sperm.  In  the  maturation  of 
the  egg,  three  out  of  the  four  cells  are  thrown  away  and  take 
no  further  part  in  the  functions  of  the  animal;  in  the  develop- 
ment of  the  sperm,  however,  each  one  of  the  four  cells  arising 
from  the  divided  spermatocyte  cell  becomes  a  typical  sperm; 
Fig.  IV.  It  is  evident  from  this  that  a  sperm  must  be  regarded, 
so  far  as  concerns  its  nuclear  matter,  as  equivalent  to  a  matured 
egg,  and  equivalent  also  to  each  of  the  three  discarded  cells 
which  have  been  thrown  away  in  the  maturation  of  the  egg. 
Both  the  sperm  and  the  egg  contain  half  the  normal  number 
of  chromosomes. 

An  examination  of  a  matured  sperm  shows  the  structure 
-'ndicated  in  Figure  121  sp.     It  consists  of  a  head,  which  is 


256  BIOLOGY 

sometimes  rounded,  but  more  commonly  elongated.  A  careful 
examination  of  this  head  shows  that  it  contains  an  equivalent 
of  the  two  chromosomes  originally  present  in  the  matured  egg. 
The  spermatozoan  head  is  therefore  really  a  nucleus.  Just 
back  of  the  head  is  a  short  piece  known  as  the  middle  piece, 
which  contains  a  centrosome.  This  is  the  smallest  part  of  the 
sperm.  The  third  part  of  the  sperm  is  the  tail,  which  is  usually 
rather  long  and  motile,  and  whose  only  function  is  to  produce 
motion  of  the  sperm  and  thus  bring  it  in  contact  with  the  egg. 
The  sperms  of  some  animals,  however,  have  no  motile  tail  and 
are  brought  into  contact  with  the  egg  by  other  means. 

The  important  conclusion  to  be  drawn  from  this  description 
of  the  origin  and  structure  of  eggs  and  sperms  is,  that  they 
are  essentially  equivalent  to  each  other.  Even  though  the  egg 
is  very  large  and  the  sperm  is  very  small,  and  though  the  egg 
is  motionless  and  the  sperm  is  commonly  endowed  with  motion, 
so  far  as  concerns,  their  most  essential  parts  they  are  identical. 
Each  contains  one  nucleus,  with  chromosomes  equivalent  to 
half  the  amount  present  in  the  ordinary  cells  of  the  organisms 
from  which  these  cells  were  derived;  each  may  contain  a  centro- 
some, though  this  is  not  always  found.  The  eggs  contain  food 
upon  which  the  young  embryo  feeds,  and  the  sperm  possesses 
a  tail  by  which  it  can  swim;  but  these  are  secondary  features, 
and  in  essential  characters  the  egg  and  sperm  are  identical. 

Entrance  of  the  Sperm  into  the  Egg. —  When  the  sperms  are 
mature  they  are  excreted  through  the  ducts  of  the  spermaries 
to  the  exterior.  If  not  excreted  into  the  water,  as  is  frequently 
the  case  with  water  animals,  a  quantity  of  liquid  is  sometimes 
excreted  with  them,  in  which  the  cells  can  swim  by  their  motile 
tail.  All  organisms  have  some  method  by  which  the  sperms 
and  eggs  are  brought  together.  Sometimes  both  of  them  are 
thrown  in  large  numbers  into  the  water  and  depend  upon 
chance  currents  to  bring  them  together.  Among  many  of  the 
higher  animals  there  are  developed  special. copulatory  organs, 
whose  function  is  to  bring  the  eggs  and  sperms  together.  Among 


SEXUAL  AND  ASEXUAL  REPRODUCTION  257 

the  endless  series  of  animals  and  plants  may  be  found  great 
variety  in  the  manner  by  which  this  is  accomplished;  but  in 
all  cases  some  efficient  device  is  found  for  bringing  the  egg 
and  sperm  into  contact. 

The  egg  and  the  sperm  have  a  strong  attraction  for  each 
other,  so  great  that  when  brought  into  each  other's  proximity 
the  sperm  will  be  attracted  to  the  egg  and  attach  itself. 
The  head  of  the  sperm  then  buries  itself  in  the  egg,  as  shown 
in  Figure  121  G,  m,  the  tail  being  left  on  the  outside,  but  the 
centrosome  being  carried  in  with  the  head.  The  tail  has  no 
further  function.  This  entrance  of  the  sperm  into  the  egg 
may  occur  either  before  or  after  the  changes  in  the  egg  that 
have  been  described  as  maturation.  If  the  sperm  enters  before 
the  egg  is  fully  matured  it  remains  in  the  egg  in  a  dormant 
condition,  and  is  now  known  as  the  male  pronucleus,  until 
after  the  egg  has  been  brought  into  the  condition  above  de- 
scribed as  mature,  with  its  chromosomes  reduced  to  half  their 
normal  number.  If  the  sperm  does  not  enter  the  egg  until 
after  the  egg  is  mature,  the  further  changes  which  bring  about 
fertilization  occur  at  once. 

Fertilization. —  After  the  sperm  has  entered  the  egg  and  the 
egg  has  become  matured,  the  nucleus  of  the  egg  and  the  sperm 
head  (the  two  pronuclei)  approach  each  other;  Fig.  121  H. 
What  brings  them  together  is  not  exactly  known;  apparently, 
in  some  cases,  the  centrosome  seems  to  have  something  to  do 
in  bringing  the  two  nuclei  in  contact,  and  without  much  doubt 
they  have  an  attraction  for  each  other.  At  all  events  the  egg 
and  the  sperm  are  soon  brought  together  and  finally  fused 
with  each  other,  forming  a  single  fusion  nucleus.  This  fusion 
is  the  fertilization  proper  (sometimes  called  impregnation). 
Since  the  egg  nucleus  contains  two  chromosomes  and  the 
sperm  head,  or  male  nucleus,  also  contains  two,  when  these 
two  unite  the  fusion  nucleus  evidently  contains  four  of  them, 
and  thus  the  number  of  chromosomes  is  restored  to  the  same 
number  as  that  possessed  by  the  ordinary  cells  of  the  body 


258  BIOLOGY 

of  the  animal.  Whether  the  centrosome  that  is  brought  in 
by  the  sperm  and  that  which  comes  from  the  egg  have  any- 
thing to  do  with  the  subsequent  history  of  the  fertilized  egg, 
is  uncertain.  In  some  cases  it  is  certain  that  the  ^centrosome 
of  the  original  egg  disappears,  and  the  only  one  that  remains 
is  the  one  brought  in  by  the  sperm.  In  plants,  as  we  have 
already  learned,  there  are  no  centrosomes  at  all,  and  from 
these  facts  it  would  seem  to  follow  that  the  centrosome  can 
not  have  very  much  to  do  with  the  process  of  fertilization. 

From  the  facts  given  it  is  evident  that  the  fertilized  egg  con- 
tains material  from  both  parents.  The  female  parent  furnishes 
the  bulk  of  the  food  in  the  egg  upon  which  the  young  is  to  be 
nourished;  and  it  also  furnishes  two  chromosomes.  The  male 
parent  has  also  furnished  two  chromosomes,  and  in  some  cases 
a  centrosome,  but  none  of  the  food  material.  The  only  thing 
which  the  two  sexes  have  furnished  in  common  is  chromatin 
material,  and  it  is  especially  interesting  to  note  that  both  the 
male  and  the  female  parent  furnish  chromosomes  in  equiva- 
lent amounts. 

Unless  an  egg  is  fertilized  by  a  sperm  it  has  no  power  of 
subsequent  growth.  Most  of  the  ordinary  cells  of  the  animal 
body  are  capable  of  a  certain  amount  of  development,  but  the 
egg  cell  if  unfertilized  soon  dies,  undergoes  decomposition  and 
disappears.  The  sperm  cell  also  is  unable  to  undergo  any 
development  by  itself.  Therefore,  the  fusion  of  an  egg  and 
a  sperm  is  necessary,  in  this  type  of  reproduction,  for  the 
development  of  a  new  individual. 

It  may  sometimes  happen  that  more  than  one  sperm  is 
brought  into  the  vicinity  of  an  egg.  When  this  occurs,  in 
most  cases  there  is  some  device  by  which  the  entrance  of 
more  than  a  single  sperm  into  the  egg  is  prevented.  In  some 
kinds  of  eggs,  it  is,  however,  not  unusual  for  more  than  one 
sperm  to  enter  the  egg,  but  when  this  occurs  only  one  of  them 
unites  with  the  egg  nucleus,  the  others  having  no  further 
function  in  the  process.  If  in  any  case  more  than  one  sperm 


SEXUAL  AND  ASEXUAL  REPRODUCTION  259 

does  unite  with  the  egg  nucleus,  abnormal  results  arise  and 
no  proper  embryo  develops.  In  the  vast  majority  of  cases, 
however,  the  single  sperm  unites  with  the  single  egg  nucleus, 
and  all  other  sperms  that  chance  to  be  present  have  nothing 
to  do  with  the  development,  but  soon  disappear. 

THE  RELATION  OF  THE  CHROMATIN  TO  HEREDITY 

The  facts  just  mentioned  show  us  that  the  chromatin  must 
play  a  very  important  part  in  the  transmission  of  characters 
from  parent  to  offspring.  It  is  a  demonstrated  fact  that 
both  the  male  and  the  female  parents  can  transmit  their 
characters  equally  to  their  offspring.  It  follows  that  both 
parents  would  probably  transmit  an  equal  amount  of  heredi- 
tary substance  to  the  next  generation.  The  process  of  fertili- 
zation just  described  shows  that  the  only  parts  contributed 
by  the  male  parent  to  the  fertilized  egg  are  the  centrosome 
and  the  chromosomes.  Hence  whatever  the  male  parent  trans- 
mits to  its  offspring  must  be  contained  either  in  the  centrosome 
or  the  chromosomes.  But  the  female  parent  does  not  contribute 
any  centrosome  to  the  combined  egg,  and  it  should  be  remem- 
bered that  in  plants  there  is  no  centrosome.  The  female  does 
contribute  an  amount  of  chromatin  equal  to  that  which  the 
male  contributes,  namely,  in  the  case  described,  two  chromo- 
somes. This  fact  proves  that  the  chromosomes  must  certainly 
contain  hereditary  material.  These  chromosomes  are  extremely 
minute,  far  below  the  reach  of  the  human  vision  and  only 
seen  with  a  high-power  microscope  and  by  special  microscopic 
methods.  It  seems  almost  incredible  that  there  can  be  in 
such  a  small  compass  the  traits  of  characters  which  an  indi- 
vidual transmits  to  its  offspring  and  which  the  offspring  in- 
herits from  its  parents.  But  the  facts  described  seem  to  be 
capable  of  no  other  interpretation,  and  we  are  therefore  justi- 
fied in  saying  that  the  chromatin  material  is  the  bearer  of 
heredity.  This  does  not  necessarily  mean  that  other  parts 
jf  the  egg  and  sperm  may  not  have  some  share  in  heredity. 


260  BIOLOGY 

The  methods  of  maturation  and  fertilization  differ  somewhat 
in  different  animals  and  plants,  but  in  all  cases  where  there 
is  the  union  of  the  egg  with  the  sperm  it  is  essentially  as  above 
described.  The  normal  number  of  chromosomes  is  first  doubled 
and  then  reduced  to  one-half  that  which  the  ordinary  cells 
of  the  organism  originally  contained.  The  mature  sperm  also 
contains  half  of  the  normal  number  of  chromosomes;  and  thus, 
when  the  egg  and  the  sperm  finally  fuse,  the  nucleus  of  the 
fertilized  egg  is  always  brought  back  into  the  original  condi- 
tion with  the  normal  number  of  chromosomes,  which  is  evi- 
dently always  an  even  number;  see  page  85. 

It  may  seem  a  little  strange  that  the  egg  should  exclude 
and  throw  away  as  useless  such  a  considerable  part  of  this 
chromatin  material,  which  must  be  of  such  great  value.  The 
reason  is  not  difficult  to  see.  If  the  egg  did  not  throw  away 
some  of  its  chromatin  material,  it  could  not  combine  with  the 
sperm  without  the  chromatin  material  in  the  combined  egg 
being  doubled  in  quantity.  If,  for  instance,  in  the  case  de* 
scribed,  the  egg  and  sperm  should  retain  their  normal  number 
of  chromosomes,  then,  after  the  egg  and  sperm  united,  the 
nucleus  of  the  fertilized  egg  would  contain  eight  instead  of 
four,  and  all  of  the  subsequent  cells  would  necessarily  contain, 
eight.  If  the  process  were  repeated  at  the  next  reproduction 
the  number  would  again  double  and  thus  the  amount  of  the 
chromatin  material  in  each  cell  would  become  greater,  genera- 
tion after  generation.  To  keep  the  number  of  chromosomes 
the  same  in  successive  generations,  both  the  sperm  and  the 
egg  throw  away  some  of  their  chromatin  to  make  room  for 
an  equal  amount  brought  in  by  the  other  cell  at  fertilization. 
Why  the  number  is  first  doubled  before  being  reduced  is  not 
clear. 

THE  PURPOSE  OF  THE  UNION  OF  THE  SEXES 

Since  sex  union  is  almost  if  not  quite  universal  among 
animals  and  plants,  it  is  evident  that  the  process  must  be 
one  of  very  great  significance.  One  of  its  purposes  is  very 


SEXUAL  AND  ASEXUAL  REPRODUCTION  261 

evident.  Inasmuch  as  the  chromosomes  contain  the  substance 
which  transfers  the  hereditary  traits,  it  follows  as  a  result  of 
this  cell  union  that  the  individual  that  is  to  arise  from  the 
fertilized  egg  will  inherit  traits  of  character,  not  from  one  but 
from  two  parents.  This  will  naturally  produce  a  greater 
variety  in  the  offspring.  If  an  individual  arose  simply  as  a 
result  of  the  division  of  a  single  parent,  it  would  be  expected 
that  it  would  have  a  tendency  to  show  a  much  greater  like- 
ness to  its  parent  than  if  it  arose  from  the  fusion  of  cells  from 
two  parents,  each  of  which  possessed  its  own  individual  char- 
acteristics. Thus,  as  a  result  of  this  sexual  union,  there  will 
be  introduced  into  the  offspring  a  tendency  toward  variety, 
which  would  hardly  be  expected  if  they  were  produced  always 
by  the  non-sexual  methods  of  simple  division.  It  is  believed 
by  biologists  that  one  purpose  of  sex  union  is  to  produce 
variety  among  organisms,  i.  e.,  to  introduce  what  is  technically 
called  variation.  The  importance  of  variation  will  be  discussed 
later;  here  it  will  be  sufficient  to  say  that  upon  the  phenomena 
of  variation  is  based  the  whole  problem  of  the  evolution  of 
animals  and  plants,  and  therefore,  without  this  phenomenon 
of  sex  union,  the  evolution  of  animals  and  plants  could  hardly 
have  taken  place,  at  least  not  in  the  form  in  which  it  has  oc- 
curred in  the  actual  history  of  living  things. 

The  process  thus  becomes  intelligible.  Sex  union  brings 
about  the  combination  in  the  offspring,  of  the  qualities  of  two 
parents,  and  thus  produces  a  succession  of  generations  which, 
though  much  alike,  still  show  a  certain  amount  of  variation 
among  themselves  and  hence  a  variation  from  the  ancestral 
type. 


CHAPTER  XIII 

DISTRIBUTION  OF  SEXUAL  AND  ASEXUAL  METHODS. 
ALTERNATION  OF  GENERATIONS 

SUMMARY  OF  THE  METHODS  OF  REPRODUCTION 

REPRODUCTION  in  all  animals  and  plants  is  the  result  of  divi- 
sion, but  according  to  whether  the  division  takes  place  with 
or  without  cell  union,  we  have  the  two  following  types: — 

1.  Asexual  reproduction. —  Asexual  reproduction  is  division 
without  cell  union.     Under  this  head  there  are  at  least  four 
different  methods. 

A.  Division  by  fission. 

B.  Division  by  budding. 

C.  Division  by  spore  formation. 

Each  of  these  three  types  of  reproduction  is  found  among 
unicellular  as  well  as  multicellular  organisms. 

D.  Parthenogenesis,  or  reproduction  by  eggs   without  fer- 
tilization. 

2.  Sexual    reproduction. —  Sexual    reproduction    is    division 
preceded  or  accompanied  by  a  union  of  cells,  the  uniting  cells 
being  called  gametes.    According  to  whether  the  uniting  cells 
are  alike  or  unlike,  we  find  tw9  types. 

A.  Conjugation. —  When  the  uniting  cells  are  microscopically 
identical  with  each  other,  the  process  is  conjugation.  In  these 
cases  there  are  neither  eggs  nor  sperms,  and  the  cell  resulting 
from  their  union  is  a  zygospore. 

Conjugation  has  apparently  for  its  purpose  the  reinvigora- 
tion  of  the  process  of  cell  division,  since,  after  two  individuals 
have  united,  cell  division  begins  to  take  place  more  rapidly. 
After  many  generations  of  simple  cell  division  the  process 
tends  to  become  slower,  and  conjugation  then  may  occur  to 

262 


DISTRIBUTION  OF  REPRODUCTIVE  METHODS         26S 

reinvigorate  the  process.  Conjugation  occurs  chiefly  among 
the  unicellular  organisms.  It  is  found  also  among  some  multi- 
cellular  plants,  but  in  no  multicellular  animals. 

B.  Fertilization,  or  sex  union  proper. —  When  the  uniting 
cells  are  unlike,  one  being  much  larger  than  the  other,  their 
union  constitutes  fertilization,  or  true  sex  union.  The  larger 
of  the  two  uniting  bodies  is  the  egg  and  the  smaller  the  sperm. 

ORIGIN  OF  SEX  UNION 

Conjugation  and  sex  union  are  evidently  closely  allied.  In- 
deed, some  organisms  show  a  type  of  reproduction  that  is 
halfway  between  conjugation  and  true  sex  union,  and  give 
us  an  idea  as  to  what  was  probably  the  origin  of  sex.  We 
have  already  studied  Pandorina  (Fig.  28),  in  which  we  found 
an  animal  multiplying  by  the  union  of  two  similar  cells; 
but  the  two  cells,  although  similar,  are  not  exactly  alike.  Both 
are  rounded  cells,  both  provided  with  flagella  which  enable 
them  to  swim;  but  one  is  a  little  larger  than  the  other,  and 
when  union  occurs  it  is  always  that  of  a  larger  with  a  smaller 
cell.  Whether  this  is  a  true  sex  union  or  a  conjugation  it  is 
difficult  to  decide. 

A  step  further  in  the  line  of  sex  differentiation  is  found  in 
Eudorina.  This  organism  is  much  like  Pandorina,  and  is 
composed  of  a  cluster  of  rounded  flagellate  cells,  inclosed  in 
jelly;  Fig.  123  A.  They  multiply  by  a  method  of  simple  divi- 
sion as  does  Pandorina  (shown  at  A),  and  in  addition  they 
multiply  by  cell  union.  In  the  latter  case  the  cells  break  up 
into  many  small  parts,  after  which  there  is  a  union  of  cells. 
But  here  the  uniting  cells  are  very  unlike.  Some  of  the  cells, 
shown  at  C,  D,  E,  break  up  into  a  large  number  of  small  flagel- 
late cells,  of  an  elongated  shape.  The  other  cells  of  the  colony 
do  not  divide,  but  slightly  enlarge  and  remain  spherical.  Even- 
tually one  of  the  small  flagellate  cells  comes  in  contact  with 
one  of  the  rounded  ones  and  the  two  unite.  Here  there  is  a 
plain  suggestion  of  egg  and  sperm,  and  consequently  of  a  true 


264 


BIOLOGY 


sex  union.  Only  one  more  step  is  needed  to  have  a  typical 
sexual  reproduction.  In  Eudorina  all  of  the  cells  of  the  colony 
share  in  the  reproductive  process.  If  only  a  few  of  the  cells 
of  the  colony  should  thus  develop  into  sex  cells,  leaving  the 
colony  to  live  an  independent  life,  even  after  the  sex  cells  have 
been  extruded,  there  would  be  a  typical  sexual  reproduction. 


B 

FIG.  123. —  EUDORINA 


D 


A,  showing  the  asexual  reproduction  by  division.  C,  D,  and  E  show  some  cells  which  are 
dividing  into  numerous  flagellate  gametes.  These  unite  with  the  larger  cells  in  B,  which 
are  thus  also  gametes. 

Such  a  condition  is  found  in  the  multicellular  plants  and  ani- 
mals generally. 

Prom  such  data  as  these  it  is  evident  that  the  probable 
origin  of  sexual  reproduction  has  been  something  as  follows: 
The  first  method  of  reproduction  was  by  simple  division,  but 
the  independent  individuals  acquired  the  habit  of  fusing  with 
each  other,  as  we  have  seen  in  the  case  of  the  Paramecium, 
this  fusion  reinvigorating  the  life  power  of  the  fused  individual. 
Next  there  was  probably  a  tendency  for  the  cells  to  break  up 
into  many  parts  which  subsequently  united  with  each  other, 
the  parts  being  at  first  all  alike.  The  next  step  seemed  to  be 
for  some  of  these  cells  to  contain  more  food  than  the  others 
and  become  larger;  this  led  to  the  larger  cells  having  less  power 
of  motion,  while  the  smaller  ones  retained  the  power.  Next 


DISTRIBUTION  OF  REPRODUCTIVE  METHODS         265 

the  larger  cells  lost  their  swimming  flagella  and  were  brought 
into  contact  with  the  smaller  cells  only  by  the  motions  of 
the  latter,  which  still  retained  their  flagella.  Lastly,  most 
of  the  cells  of  an  organism  ceased  to  have  any  share  in 
reproduction,  being  simply  concerned  in  the  life  of  the  colony. 
Some  of  the  cells  in  such  a  colony,  however,  assumed  as  their 
part  the  process  of  uniting  with  others,  and  thus  carried  on 
the  functions  of  reproduction.  These  cells  still  continued  to 
differentiate  into  large  and  small  cells,  the  large  ones  becoming 
eggs  and  the  small  ones  remaining  as  sperms.  From  this 
time  on  the  function  of  reproduction  is  independent  of  the 
functions  of  the  life  of  the  colony,  and  the  individual  exists 
apart  from  its  offspring.  From  all  of  this  it  appears  that  con- 
jugation is  the  first  step  in  the  direction  of  sex  union,  and 
that  conjugation  must  therefore  be  regarded  as  a  form  of  sex 
union,  although  the  sexes  have  not  been  sharply  differentiated 
in  any  true  case  of  conjugation. 

DISTRIBUTION  OF  ASEXUAL  REPRODUCTION 

Among  plants  asexual  reproduction  is  nearly  universal, 
all  of  the  lower  plants,  and  nearly  all  the  higher  ones,  being 
able  to  multiply  by  some  form  of  budding  or  division.  Par- 
thenogenesis is  also  fairly  frequent.  Among  animals  multi- 
plication by  budding  or  division  is  also  widely  distributed. 
It  is  universal  among  the  unicellular  animals,  and  is  a  common 
method  of  multiplication  among  such  lower  forms  as  Hydra 
and  its  allies.  As  we  pass  to  higher  animals  this  power  dis- 
appears. It  is  found  among  some  worms,  and  one  group  of 
animals  related  to  the  vertebrates  (Tunicata)  forms  colonies 
by  budding,  which  may  break  up  and  become  several  colonies, 
this  constituting  a  modified  kind  of  reproduction.  In  no  other 
higher  animals  does  asexual  reproduction  occur.  The  modified 
type  of  asexual  reproduction  which  is  called  parthenogenesis 
is  found  among  some  of  the  higher  animals,  being  fairly  com- 
mon even  among  insects. 


266  BIOLOGY 

DISTRIBUTION  OF  SEXUAL  REPRODUCTION 

Sexual  reproduction,  using  this  term  to  include  conjugation, 
is  very  widely  distributed  among  organisms  and,  indeed,  is 
possibly  coextensive  with  life.  It  is  true  that  there  are  many 
forms  of  unicellular  animals  and  plants  in  which  it  has  never 
been  shown  to  occur;  but  in  many  cases  this  is  due  to  incom- 
plete knowledge.  With  increasing  knowledge,  more  and  more 
of  the  unicellular  organisms  are  known  to  go  through  the  proc- 
ess of  cell  union  under  some  conditions.  Even  some  of  the 
longest  known  and  best  studied  organisms  (Amoeba)  have 
been  recently  shown  to  undergo  conjugation.  Among  some 
of  the  unicellular  forms,  too,  there  occurs  a  true  sexual  union. 
In  the  malarial  organism,  for  example,  there  is  at  one  stage 
in  the  life  history  a  union  of  two  unlike  cells,  which  are  regarded 
as  male  and  female,  and  a  similar  differentiation  of  uniting 
bodies  has  been  found  in  many  other  single-celled  organisms. 
The  continued  discovery  of  new  examples  of  sexual  union  or 
conjugation,  among  the  lower  organisms  previously  supposed 
not  to  have  this  power,  has  led  to  a  belief  that  a  union  of  cells 
in  reproduction  may  be  a  universal  characteristic  of  all  life, 
even  though  there  are  still  many  of  the  lower  animals  and 
plants  in  which  it  has  not  been  found.  This  conclusion  is 
as  yet  by  no  means  proved  and  may  not  turn  out  to  be  strictly 
true.  In  all  groups  of  animals  above  the  unicellular  types, 
sexual  reproduction,  by  the  union  of  true  male  and  female 
cells,  is  universal,  and  in  the  higher  groups  it  is  the  only  method 
of  multiplication  known  to  occur. 

REPRODUCTIVE  BODIES  OR  REPRODUCTIVE  CELLS 

This  term  refers  to  the  parts  which  are  separated  from  the 
bodies  of  animals  or  plants,  and  capable  of  growing  into  new 
individuals.  Sometimes  they  are  multicellular  fragments,  like 
the  buds  of  Hydra  or  the  gemmce  of  a  plant;  but  in  such  cases 
the  term  reproductive  body  is  not  usually  applied  to  them. 
In  the  large  majority  of  cases  the  bodies  formed  for  reproductive 


DISTRIBUTION   OF  REPRODUCTIVE  METHODS        267 

purposes  are  single  cells  which  are  capable  of  developing  into 
new  individuals,  and  hence  the  term  reproductive  cells  better 
describes  them.  Of  these  reproductive  cells  we  recognize  the 
following  kinds :  — 

Spores:  single-celled  reproductive  bodies,  capable  of  growing 
into  new  organisms  without  uniting  with  a  sperm. 

Eggs,  or  ova:  large,  stationary  cells,  which  grow  into  new 
individuals  only  after  uniting  with  a  sperm. 

Sperms:  minute,  usually  motile  cells,  which  must  unite  with 
an  egg  to  enable  it  to  develop. 

Parthenogenetic  eggs:  large,  stationary  cells,  resembling,  or 
identical  with,  eggs,  but  able  to  develop  without  union  with 
a  sperm. 

The  name  gametes  (Gr.  gamete  =  wife  or  husband)  is  fre- 
quently applied  to  the  cells  that  unite  with  each  other  in  cell 
union.  This  term,  therefore,  includes  eggs  and  sperms,  and  also 
the  uniting  cells  in  conjugation  where  no  distinction  of  sex 
is  seen. 

CROSS  FERTILIZATION  THE  RULE 

Cross  Fertilization. —  In  ordinary  sexual  reproduction  the 
rule  is  that  a  single  sperm  unites  with  a  single  egg.  When 
the  sexes  are  separate,  as  in  the  frog,  this  will  always  result 
in  the  fertilization  of  an  egg  from  one  individual  with  a  sperm 
from  another.  As  we  have  seen,  some  animals  produce  both 
eggs  and  sperms,  and  might  fertilize  their  own  eggs.  But 
usually  there  is  some  device  to  prevent  this.  In  the  earth- 
worm, although  both  eggs  and  sperms  are  produced  by  each 
individual,  in  copulation  there  is  an  interchange  of  sperm 
fluid,  in  such  a  way  that  the  eggs  of  each  individual  are  sub- 
sequently fertilized  by  the  sperms  from  the  other.  This  is 
called  cross  fertilization.  In  most  cases  where  both  male  and 
female  organs  are  produced  in  the  same  individual,  there  is 
some  device  by  which  cross  fertilization  is  insured.  In  the 


268  BIOLOGY 

common  flowers  both  male  and  female  organs  are  developed 
in  each  flower,  but  there  is  almost  always  some  means  which 
prevents  the  flower  from  self-fertilization  and  insures  cross 
fertilization.  In  a  few  animals  and  plants,  it  is  true,  self-fertil- 
ization appears  to  be  the  rule,  but  it  is  very  unusual. 

It  appears  that  the  reason  why  cross  fertilization  is  so  com- 
monly found,  is  that  it  results  in  more  or  stronger  offspring. 
Experiments  carefully  carried  out  in  plants  have  shown  that, 
in  many  cases  at  least,  the  offspring  resulting  from  cross  fertil- 
ization are  more  vigorous  than  those  coming  from  close  fertil- 
ization. In  animals  there  is  less  evidence  at  hand  on  the 
subject,  but  here,  too,  it  has  been  recently  shown  that,  in  some 
cases  at  all  events,  cross  fertilization  is  more  productive  of  a 
vigorous  progeny.  Apparently,  then,  cross  fertilization  is  based 
upon  a  fundamental  law. 

Hybrids. —  On  the  other  hand,  it  is  necessary  that  the  sperm 
that  unites  with  the  egg  shall  come  from  another  individual 
not  too  unlike  the  one  that  produces  the  egg.  If  the  egg  be- 
longs to  one  species  of  animal  or  plant  and  the  sperm  to  another 
species,  they  are  not  likely  to  unite  at  all.  If  two  different  spe- 
cies are  crossed  the  rule  is  that  there  is  no  offspring,  or  that, 
if  there  is  offspring,  they  will  themselves  be  incapable  of  pro- 
ducing young.  Such  an  individual  is  known  as  a  hybrid, 
and  frequently  hybrids  are  sterile.  It  was  at  one  time  sup- 
posed that  they  were  always  sterile,  a  conclusion  that  was 
based  largely  upon  the  fact  that  the  mule,  which  is  a  hybrid 
between  a  horse  and  an  ass,  is  well  known  to  be  incapable  of 
breeding.  But  most  careful  study  of  both  animals  and  plants 
has  shown  many  instances  where  hybrids  are  fertile,  so  that 
the  sterility  of  hybrids  is  by  no  means  a  fixed  rule.  In  general, 
however,  in  order  to  produce  the  most  vigorous  offspring  it  is 
necessary  that  the  eggs  of  one  individual  should  be  fertilized 
by  sperms  from  another  individual  of  the  same  species,  but 
not  too  closely  related.  Close  inbreeding  has  a  tendency  to 
foster  weakness. 


REPRODUCTION:  ALTERNATION  OF  GENERATIONS      269 


ALTERNATION  OF  SEXUAL  WITH  ASEXUAL  METHODS  OF 
REPRODUCTION 

In  many  plants,  and  in  some  animals,  there  is  a  regular 
alternation  in  the  methods  of  reproduction,  i.  e.,  that  with 
sex  union  and  that  without  sex  union.  This  is  commonly 
spoken  of  as  the  al- 
ternation of  genera- 
tions. One  of  the 
simplest  and  most 
easily  understood  ex- 
amples is  in  that  of 
the  common  fern. 

Life  History  of  the 
Fern. —  At  certain 
seasons  of  the  year, 
usually  in  the  fall, 
there  appear  upon  the 
under  surface  of  the 
fern  leaves,  or  fronds, 
which  grow  every- 
where by  the  road- 
side, little  rounded 
disks  known  as  son; 
Fig.  124  B.  They 
are  sometimes  cov- 
ered by  a  little  scale 
called  an  indusium. 
A  study  of  these 
disks  with  a  micro- 
scope shows  that  they 
are  made  up  of  a  number  of  little  sacs,  containing  minute 
reproductive  bodies;  Fig.  D.  When  mature  the  sacs  burst 
and  the  reproductive  cells  are  thrown  out  into  the  air.  If 
they  fall  upon  some  surface  where  they  have  the  proper  tem- 
perature and  moisture,  they  begin  to  grow  at  once;  and  since 


FIG.  124. —  COMMON  FERN 

A,  the  fern  attached  to  its  root-stock;  B,  the  back 
of  two  leaflets,  showing  the  sori ;  C,  a  leaflet  more  highly 
magnified  showing  the  sporangia  within  the  sori;  D,  one 
of  the  sporangia  still  more  highly  magnified  discharging 
spores. 

s,  sori; 

sp,  spor.es; 

spg,  sporangia. 


270 


BIOLOGY 


they  are  thus  capable  of  growing  immediately  into  new  plant! 
without  being  united  with  sperms,  we  know  that  they  must 
be  spores  and  not  eggs,  since  eggs  require  fertilization  before 
they  will  develop.  The  sacs  that  contain  them  are  sporangia, 
spg.  This  method  of  reproduction  is  therefore  evidently  an 
asexual  method. 

When  these  spores  develop  they  do  not,   however,   grow 
into  a  plant  like  the  original  fern,  but  each  grows  into  a  very 


ar 


x// 


FIG.  125 — THE  LIFE  HISTORY  OP  THE  FERN 

A  and  B,  sprouting  spore;  C,  prothallium  full  grown;  D,  section  of  an  archegonium;  E, 
archegonium  at  a  later  stage,  showing  the  ovum,  o,  and  the  sperm,  spm,  entering  to  fertilize 
the  ovum;  F,  section  of  an  antheridium  at  an  early  stage;  G,  an  antheridium  at  a  later  stage, 
discharging  sperms;  H,  the  young  fern,  /,  growing  from  its  prothallium. 

small,  flat,  green  leaf  (Fig.  125  A  to  C),  which  clings  closely 
to  the  ground,  as  shown  at  H ,  usually  not  growing  to  more 
than  one-quarter  inch  in  diameter,  and  frequently  even  less. 
It  has  no  stem,  but  on  the  under  surface  are  a  few  delicate 
hairs  called  rhizoids,  which  grow  downward,  fastening  the 


REPRODUCTION:  ALTERNATION  OF  GENERATIONS    271 

plant  to  the  soil  and  giving  it  nourishment.     It  is  called  a 
prothallium  (Lat.  pro  =  before  +  ihallus  =  branch)   and  one 
would  never  suspect  that  this  little  plant  had  anything  to  do 
with  the  fern  which  produced  it.    We  rarely  see  the  prothallia 
of  the  fern,  not  because  they  are  not  abundant,  but  because 
they  are  so  small  and  grow  so  closely  to  the  ground  that  they 
do  not  attract  attention.     They  may  be  found  without  much 
difficulty,  however,  by  carefully  searching  for  them.     One  of 
the  easiest  places  to  find  them  is  on  the  outside  of  the  moist 
earthen  flower  pots  in  a  greenhouse  where  ferns  are  abundant. 
After  the  prothallium  has  reached  its  full  growth  an  exami- 
nation of  its  under  surface  with  a  microscope  shows  that  it 
in  turn  is  getting  ready  to  carry  on  a  process  of  reproduction. 
On  the  under  surface  may  be  found  several  little  projections 
(Fig.  C),  too  small  to  be  visible  to  the  naked  eye  but  clearly 
made  out  with  the  microscope.     They  are  of  two  kinds,  one 
lying  among  the  rhizoids  near  one  edge  of  the  leaf,  an,  and  the 
other  lying  near  the  other  edge,  some'  distance  from  the  rhi- 
zoids, ar.     The  latter  are  slightly  elongated,  with  an  opening 
at  the  free  end,  and  a  little  canal  extending  down  the  middle: 
they  are  called  archegonia  (Gr.  arche-  =  beginning  +  gonos  = 
birth) ;  Figs.  D  and  E.    At  the  base  of  each  archegonium  is  a 
single  egg,  o.    The  other  protuberances,  lying  near  the  edge  of 
the  leaf  among  the  rhizoids,  are  called  antheridia  (Gr.  antheros 
=  flowery) ;  Figs.  F  and  G.    They  are  more  rounded  in  shape, 
not  so  long  as  the  archegonia,  and  their  contents   are   quite 
different.     Instead  of  containing  a  single  egg,  the  whole  con- 
tents of  an  antheridium  divides  up  into  a  large  number  of 
parts.    Eventually  an  opening  makes  its  appearance  at  the  end 
of  the  antheridium,  and  these  minute  bodies  emerge  and  prove 
to  be   sperms,  spm   (sometimes  called  spermatozoids).     The 
fern   prothallium  grows  only  on    moist    surfaces    and    clings 
so  closely  to  the  ground  that  in  times  of  rains  or  heavy  dew 
its  under  surface  is  likely  to  be  covered  with  water.     Each 
sperm  bears  a  tuft  of  swimming  flagella,  which  lash  to  and  fro 


272  BIOLOGY 

and  enable  them  to  swim  in  the  water,  which  moistens  the 
under  surface  of  the  prothallium.  In  thie  moisture  they  swirn 
in  all  directions,  and  some  of  them  come  to  the  mouths  of  the 
archegonia.  When  this  occurs  there  is  an  attraction  between 
the  egg  at  the  bottom  of  each  archegonium  and  the  sperm  which 
has  reached  its  top;  the  sperm  swims  to  the  egg  and  fuse:- 
with  it,  i.  e.,  fertilizes  it. 

After  the  egg  has  been  fertilized  by  the  sperm,  it  is  endowed, 
like  any  other  fertilized  egg,  with  the  power  of  growth.  It 
soon  begins  to  divide,  grows  rapidly,  and  develops  in  the  course 
of  time  into  a  little  plant  which,  by  continued  growth,  becomes 
the  fern  with  which  we  are  familiar  and  like  that  with  which  we 
started  the  history;  Fig.  H,  f.  Thus  we  see  that  the  common 
fern  grows  from  a  fertilized  egg,  and  that  the  spore  produced 
by  the  fern  grows,  not  into  a  fern  at  first,  but  rather  into  a 
prothallium. 

The  life  history  of  the  fern  is  thus  an  alternation  of  two 
different  stages  and  two  different  methods  of  reproduction. 
There  is  first  the  fern  proper,  which,  since  it  produces  only 
spores,  is  the  asexual  stage  of  the  plant,  and  is  called  the  sporo- 
phyte  (Gr.  sporos  +  phyton  =  plant).  The  second  stage  is  the 
prothallium,  which,  since  it  produces  eggs  and  sperms,  is  the 
sexual  stage.  This  is  called  the  gametophyte  (Gr.  gamete  -f- 
phyton)  stage,  since  it  produces  gametes.  Each  of  the  thou- 
sands of  spores  of  the  fern  is  capable  of  producing  a  single 
prothallium.  The  single  egg  at  the  bottom  of  each  arche- 
gonium is  capable  of  developing  a  single  fern,  and  since  there 
are  several  archegonia  on  each  prothallium,  a  prothallium  is 
thus  theoretically  able  to  produce  several  ferns.  Usually, 
however,  only  one  of  the  eggs  becomes  fertilized  by  a  sperm, 
therefore  only  a  single  fern  develops  from  a  prothallium.  Some- 
times two  eggs  may  grow,  and  occasionally  three  may  develop, 
so  that  two  or  three  little  ferns  may  sometimes  be  found  grow- 
ing from  a  single  prothallium. 

Alternation  of  Generations  in  a  Flowering  Plant. —  In  a 


REPRODUCTION:  ALTERNATION  OF  GENERATIONS      273 

common  flowering  plant  there  is  an  alternation  of  generations, 
based  upon  the  same  principle  as  that  just  described  in  the 
fern;  but  it  is  so  obscured  by  certain  modifications  that  it  is 
extremely  difficult  to  understand.  The  difficulty  lies  in  three 
facts:  (1)  Two  kinds  of  spores  are  produced  instead  of  one,  as 
in  the  fern;  one  of  them  becomes  the  female  gametophyte, 
;  producing  the  equivalent  of  the  archegonium  of  the  prothallium 
Vth  its  egg,  while  the  other  becomes  a  male  gametophyte, 
producing  the  equivalent  of  the  antheridium  of  the  prothallium 
with  its  sperms.  (2)  Both  of  these  gametophytes  have  become 
very  much  reduced  in  size  and  are  only  distinguishable  by  micro- 
scopic examination  with  special  methods.  (3)  These  two 
gametophytes  grow  attached  to  the  plant  that  produces  the  spores 
instead  of  detached  from  it,  as  does  the  gametophyte  of  the 
fern.  If  these  differences  be  kept  in  mind  the  alternation  of 
generations  in  the  flowering  plant  is  plain.  It  is  as  follows:  — 
We  usually  speak  of  the  flower  as  containing  sexual  organs, 
the  stamens  being  spoken  of  as  the  male  and  the  pistil  as  the 
female  organs.  When  the  pollen  is  carried  to  the  pistil  it  has 
commonly  been  spoken  of  as  fertilizing  the  stigma,  the  infer- 
ence being  that  the  pollen  is  the  male  cell  and  actually  fertil- 
izes the  female  cell  in  the  pistil.  When  the  flower  is  studied 
by  modern  methods,  however,  it  is  found  that  in  reality  it  is 
not  a  sexual  plant  at  all,  and  does  not  produce  sexual  organs. 
The  stamens  are  not  male  organs  and  the  pollen  is  not  a  male 
cell;  the  pistil  itself  produces  no  eggs.  The  pollen  is  really  a 
mass  of  spores,  called  microspores.  In  the  pistil,  as  already 
noticed  (see  Fig.  64),  are  several  ovules  and  inside  of  each 
ovule  is  a  single  large  cell,  formerly  called  the  embryo  sac,  but 
now  known  as  a  macrospore ;  Fig.  126  sp.  The  flower  thus 
produces  large  numbers  of  microspores  and  a  smaller  number 
of  macrospores,  which  together  correspond  to  the  spores  of 
the  fern.  These  cells  are  known  to  be  spores  rather  than 
gametes,  since  they  do  not  unite  with  each  other.  That 
ihe  pollen  is  a  spore  rather  than  a  sex  cell  is  proved  by 


274  BIOLOGY 

the  fact  that  it  will  grow  into  a  new  plant  without   being 
united  with  another  cell.     The  macrospore  is  also  proved  to 

be  a  spore  by  the  same  fact,  since  it 
also  grows  into  a  new  plant  without  be- 
ing fertilized.     Since  the  flower-bear- 
ing plant  thus  produces  spores  instead 
of  eggs  and   sperms,   it   is   clearly  a 
sporophyte  rather  than  a  gametophyte, 
and  it  corresponds  to  the  fern  frond 
rather  than  the  fern  prothallium.     It 
FIG.  126.  — MAGNIFIED      differs  from  the  fern,  however,  in  that 
SECTION  OF  THE  YOUNG     it  produces  two  kinds  of  spores  instead 
OVULE,  o,  OF  A  FLOWER-     of  one.    This  condition  is  spoken  of  as 
ING  PLANT  heterosporous    (Gr.  heteros  ••=  other  + 

sp,  macrospore;  n,  its  nucleus.        ^^    in  digtinction    from    the    homo. 

sporous  (Gr.  homos  =  alike)  condition  of  the  fern. 

If  we  now  try  to  follow  out  a  comparison  between  the  flower 
and  the  fern,  we  should  expect  that  the  flower  spores  would 
germinate  at  once  into  gametophytes,  just  as  the  fern  spores 
germinate  into  the  prothallium,  and  that  the  gametophytes 
would  produce  the  real  sex  organs  with  sperms  and  eggs.  Since, 
however,  there  are  two  kinds  of  spores,  we  might  expect  two 
kinds  of  gametophytes  to  grow  from  them  instead  of  one  kind, 
as  in  the  fern.  This  actually  occurs,  only  the  two  gametophytee 
are  very  small  and  rudimentary.  The  macrospore  never  gets 
out  of  the  pistil  but,  in  the  midst  of  the  pistil  tissue,  develops 
quickly  into  a  tiny  growth  that  represents  a  gametophyte 
stage,  and  this  soon  produces  what  corresponds  to  an  arche- 
gonium  with  its  egg;  Fig,  127.  All  this  occurs  early  in  the 
life  of  the  flower,  before  any  pollen  has  been  brought  to  the 
pistil,  and  consequently  before  fertilization  can  have  occurred. 
It  is  simply  the  germination  of  a  spore  to  form  a  gametophyte. 
The  pollen,  too,  goes  through  its  history,  growing  very  slightly 
but  sufficiently,  to  show  that  it  develops  into  a  gametophyte 
in  its  turn.  This  occurs  either  before  it  has  left  the  anther 


REPRODUCTION:  ALTERNATION  OF  GENERATIONS      275 


that  produced  it,  or  after  it  has  been  ca-ried  to  the  pistil. 
The  growth  of  the  pollen,  as  well  ar>  its  i  ^semblance  to  the 
gametophyte,  is  so  slight  that 
it  was  not  recognized  for  years 
after  plants  had  been  carefully 
studied  But  it  is  now  known 
thai  the  pollen  does,  at  least 
in  some  of  the  higher  plants, 
develop  sufficiently  to  show  the 
gametophyte  stage  and  then 
produces  what  corresponds  to 
antheridia;  Fig.  128  g.  The 
pollen  tube  which  grows  down 
through  the  style  of  the  pistil 
(Figs.  65  and  127  pi),  in  a  way 
corresponds  to  the  antheridium ; 
and  inside  it  are  small  cells,  or 
nuclei  of  cells,  m,  that  corre 
spondtoand  have  thesame  func- 
tion as  sperms.  In  other  words, 
the  pollen  does  not  correspond 
to  a  sperm,  but  is  simply  a  spore 
that  grows  into  a  male  gameto- 
phyte, which  itself  produces  the 
equivalent  of  sperms. 

It  is  thus   seen  that  inside 
of  the  pistil  one  kind  of  spore 

grows  into  a  female  gametophyte  and  produces  eggs,  while  on 
the  stigma  the  other  kind  of  spore  grows  into  a  rudimentary 
male  gametophyte  and  produces  the  equivalents  of  sperms. 

Following  farther  the  comparison  with  a  fern,  the  next  step 
is  the  fertilization  of  the  egg  of  the  female  gametophyte  by 
the  sperm  of  the  male  gametophyte.  In  the  flower  this  fusion 
is  accomplished  as  follows:  The  pollen  tube  "(Fig.  1281?)  is  an 
outgrowth  from  the  male  gametophyte,  and  pushes  its  way 


FIG.  127. —  A  SECTION  OP  AN 

OVULE  AFTER  THE  SPORE  HAS 
GROWN  INTO  THE  FEMALE  GAME- 
TOPHYTE 

G,  the  gametophyte;  e,  egg;  pt,  a  pollen 
tube  pushing  its  way  through  the  style  to 
fertilize  the  egg;  m,  is  the  male  nucleus  in 
the  pollen  which  corresponds  to  the  sperm 
and  fertilizes  the  egg. 


276 


BIOLOGY 


down  the  style  until  it  reaches  the  ovule  at  the  bottom  of  the 
ovary;  see  Figs.  65  and  127  pt.  In  this  ovule  the  female 
gametophyte  has  formed,  and  has  by  this  time  produced  what 
corresponds  to  archegonia  with  their  eggs;  Fig.  127  e.  The  tip 
of  the  pollen  tube  approaches  the  egg  and  finally  comes  in 
contact  with  it.  Inside  o*  the  pollen  tube  are  nuclei  which 
represent  the  sperms;  Fig.  127  m.  As  we  have  noticed  on 
page  257,  when  the  fertilization  of  an  egg  occurs  it  is  only  the 
nuclei  of  the  cells  that  fuse,  so  that  the  nuclei  in  the  pollen 


FIG.  128. —  DEVELOPMENT  OF  THE  POLLEN 

A,  a  single  pollen  grain  or  microspore;  B,  the  cell  divided  into  two;  C,  the  pollen,  which 
has  produced  a  rudimentary  gametophyte  at  g;  D,  a  later  stage  with  the  gametophyte  g 
still  more  rudimentary;  E,  the  pollen  developing  the  pollen  tube.  The  nucleus  m 
divides  later  into  two  nuclei  representing  sperms. 

tube  represent  all  of  the  important  parts  of  a  sperm.  When 
the  pollen  tube  comes  in  contact  with  the  egg  it  allows  these 
nuclei  to  escape  into  the  egg,  where  one  of  them  fuses  with  the 
nucleus  of  the  egg,  thus  producing  the  actual  sex  union. 

The  fertilized  egg  is  now  endowed  with  powers  of  growth 
and  begins  at  once  to  develop  into  a  new  plant.  Again  follow- 
ing the  comparison  with  the  fern,  we  shall  expect  that  the  plant 
which  comes  from  the  fertilized  egg  must  be  the  sporophyte, 
which  in  this  case  is,  of  course,  the  plant  that  produces  the 
flowers.  The  egg  develops  at  once,  growing  quickly  into  a 
tiny  plant  with  a  stem  and  one  or  two  leaves.  This  occurs 
while  the  egg  is  still  retained  in  the  ovary  of  the  flower  that 
produced  the  spores.  After  a  time  this  plant  stops  growing 


REPRODUCTION:  ALTERNATION  OF  GENERATIONS    277 

and  becomes  surrounded  by  a  hard  shell,  inside  of  which  it 
remains  dormant  for  an  indefinite  period.  This  forms  the 
seed,  which  thus  appears  to  be  a  little  sporophyte  surrounded 
by  a  shell,  and  it  remains  dormant  until  later  when  it  can  be 
placed  under  proper  conditions  for  germination;  Fig.  66.  It 
develops  its  spores,  of  course,  after  it  has  grown  large  enough 
to  produce  flowers. 

It  is  thus  seen  that  the  flowering  plant  has  an  alternation 
of  generations  as  truly  as  does  the  fern,  only  in  the  flowering 
plant  the  sex  stage,  the  gametophyte,  is  very  small,  while  the 
asexual  stage  is  very  large.  The  plant  with  which  we  are 
familiar  is  in  the  sporophyte  stage,  and  the  pollen  and  the 
single  cell  inside  its  ovule  are  its  spores.  These  develop  into 
tiny  growths  that  correspond  to  the  gametophytes  and  are 
developed  within,  or  attached  to,  the  sporophyte  that  produced 
the  spores,  i.  e.,  in  the  ovary  or  attached  to  the  stigma.  But 
tiny  as  they  are,  they  produce  the  equivalents  of  eggs  and 
sperms,  which  subsequently  fuse  by  true  fertilization.  The  real 
fertilization  of  the  plant,  then,  is  the  fusion  of  the  male  cell 
contained  in  the  pollen  tube  with  the  egg  contained  in  the 
ovule.  The  term  fertilization,  which  has  been  commonly  ap- 
plied to  the  transfer  of  the  pollen  to  the  stigma,  is  a  misnomer, 
and  is  largely  given  up,  the  term  pollination  being  substituted 
instead. 

Alternation  of  Generations  among  Animals. —  An  alternation 
of  generations  also  occurs  in  the  animals  known  as  hydroids, 
animals  related  to  the  Hydra.  The  fresh-water  Hydra,  as 
described  in  Chapter  VII,  multiplies  by  budding;  but  as  fast 
as  the  buds  are  produced  they  break  away  from  the  original 
animal  and  become  independent.  In  the  marine  Podocoryne, 
the  buds  do  not  break  away  but  remain  attached  to  form  a 
colony,  made  up  of  large  numbers  of  individuals;  Fig.  129. 
The  individuals  are  partially  independent  of  each  other  and. 
if  broken  apart  are  capable  of  living  independent  lives.  This 
stage  of  the  life  of  the  animal,  since  it  has  an  asexual  multi- 


278 


BIOLOGY 


plication  by  budding,  is  the  asexual  stage,  and  is  comparable 
to  the  asexual  stage  of  the  fern  above  described  (the  fern 
proper).  It  differs  from  the  fern,  however,  in  the  fact'  that  ?t 
does  not  produce  new  individuals  by  spores,  but  by  budding. 
After  a  colony  reaches  a  certain  stage  in  its  growth,  some 
buds  arise  which  differ  in  shape  from  the  others.  These  (Fig. 
129  gb)  are  rounded,  and  eventually  break  away  from  the 


FIG.  129. — A  COLONY  OF  HYDROIDS  (PODOCORYNE),  SHOWING  AN  ALTER- 
NATION OF  GENERATIONS 

The  feeding  animals  have  tentacles;  gb,  the  generative  buds,  which  eventually  break  away 
and  become  medusae;  m,  medusa;  mo,  mouth  of  the  jellyfish;  ov,  ovaries. 

Colony  and  assume  an  independent  existence.  These  free  buds 
now  become  bell-shaped  individuals  of  clear,  transparent 
jelly,  and  are  known  as  jellyfishes  or  medusae,  m.  The  jelly- 
fishes  have  muscles  which  enable  them  to  swim  and  travel 
for  long  distances  in  the  ocean.  As  they  have  a  mouth 
and  digestive  cavity  they  can  procure  their  own  food,  and  grow, 
frequently  attaining  considerable  size  after  separating  from  the 
original  colonies;  some  species,  indeed,  assume  a  size  very 
much  larger  than  the  animal  that  produced  them.  After  hav- 
ing lived  this  free  life  for  a  time,  each  becomes  sexually  mature, 
developing  sexual  glands,  either  ovaries  or  spermaries;  Fig.  130  g. 
The  sex  bodies  become  mature,  and  are  extruded  into  the  water, 


REPRODUCTION:  ALTERNATION  OF  GENERATIONS      279 


where  they  float  around  until  the  eggs  and  the  sperms  come 
in  contact  and  fuse,  producing  a  typical  fertilization.  The 
jellyfish  itself,  after  it  has  extruded  the  sex  bodies,  has  no 
further  function,  and  dies.  The 
egg,  however,  now  grows  into 
a  new  colony  like  the  origi- 
nal. This  jellyfish  is  evi- 
dently the  sexual  stage  in  the 
development  of  the  hydroid, 
and  corresponds  to  the  sexual 
stage  in  the  development  of 
the  fern  (the  prothallium). 

The  alternation  of  a  sexual 
with  a  non-sexual  method  is 
far  more  common  among 
plants  than  among  animals. 
It  is  developed  in  all  plants 
except  the  lower  orders,  even 
the  flowering  plants,  as  we 
have  just  seen,  having  such  an 
alternation.  Among  animals, 
however,  alternation  of  genera- 
tions is  found  only  in  the  lower  orders.  It  is  common  among 
the  Hydroids,  and  a  modified  form  of  it  occurs  in  one  of  the 
higher  animals  (Salpa) ;  but  among  the  great  majority  of  ani- 
mals, when  sexual  reproduction  is  developed,  the  non-sexual 
method  is  totally  lost. 


FIG.  130. —  A  FULL-GROWN 

JELLYFISH 
m,  mouth;  g,  gonads. 


CHAPTER   XIV 

DEVELOPMENT  OF  THE  FERTILIZED  EGG 

EMBRYOLOGY  AND  METAMORPHOSIS 

BY  the  term  embryology  is  meant  that  part  of  the  life  his- 
tory of  the  animal  or  plant  which  begins  with  the  fertilization 
of  the  egg  and  continues  up  to  the  time  when  a  developed 
animal  is  formed,  ready  to  emerge  from  the  egg  as  a  free-living, 
independent  individual.  When  it  hatches  from  the  egg  it  is 
sometimes  like  the  adult,  except  in  size;  but  sometimes  it  is 
unlike  its  parents  and  goes  through  a  further  series  of  changes. 
In  this  case  we  speak  of  these  later  stages  as  constituting  the 
larval  history  or  a  metamorphosis  (Gr.  meta  =  beyond  +  mor- 
phe  =  form).  The  development  of  animals  from  the  egg  to 
the  adult  stage,  embryology  and  metamorphosis,  has  proved 
to  be  an  especially  interesting  phase  of  biological  study,  and 
has  received  much  attention  in  the  last  fifty  years.  The  em- 
bryology of  different  animals  and  plants  differs  widely,  but 
certain  fundamental  laws  and  rules  are  found  to  apply  to  all 
alike.  In  this  introductory  study  it  is  only  possible  to  give 
a  few  of  the  fundamental  principles,  using  a  single  animal  as 
an  illustration.  For  this  purpose  will  be  described  the  de- 
velopment of  the  frog,  which,  although  peculiar  in  some 
respects,  will  illustrate  the  important  laws  both  of  embryology 
and  metamorphosis.  The  embryology  of  plants  has  also  been 
studied  rather  extensively,  but  has  not  hitherto  yielded  so 
many  interesting  lessons  as  the  embryology  of  animals. 

EMBRYOLOGY  OF  THE  FROG 

1.  Segmentation. —  The  life  of  an  individual  frog  may  be 
said  to  begin  the  instant  that  the  nucleus  of  the  egg  fuses  with 
the  head  of  the  sperm  (Fig.  121  H),  the  time  of  fertilization 
being  thus  a  starting  point  of  a  new  life.  This  fertilization  of 

280 


DEVELOPMENT  OF  THE  FERTILIZED  EGG  281 

an  egg  nucleus  seems  to  endow  it  with  renewed  power.     The 

nucleus  of  the  egg  previous  to  fertilization  has  lost  its  power  of 

division,   and   if  left  to  itself,  eventually  dies 

and    disappears;    but    after   fusing  with   the 

sperm   the   combined    nucleus   shows   a  rein- 

vigorated  power  of  growth.     It  begins  almost 

at  once  to  divide  in  two  parts  (Fig.  132  A); 

the  process  of  the  division  of  the  nucleus  fol-          A  £j 

lowed  by  the  division  of  the  cell  is  identical 

with  that  described  on  page  85.     As  a  result 

of  this  division  there  are  produced  two  cells,     FlG    131 

each  with  a  centrosome,  each  with  its  PRODUCTIVE 
nucleus,  which  contains  the  same  number  of  CELLS  OF 
chromosomes  as  the  fertilized  egg  nucleus.  FROG 
Moreover,  at  the  beginning  of  the  division,  sp^r'megg;  B'  the 
each  chromosome  is  split  lengthwise,  and  half 
of  each  chromosome  passes  into  each  of  the  two  nuclei 
of  the  two  new  cells.  Each  of  the  two  cells  thus  contains 
chromatin  material  from  each  of  the  chromosomes  of  the 
fertilized  egg,  and  since  these  chromosomes  come  partly  from 
the  male  and  partly  from  the  female  parent,  it  follows  that 
one-half  of  the  chromatin  in  each  cell  is  derived  from  the 
male,  and  one-half  from  the  female  parent.  Hence,  each  cell 
will  contain  inherited  traits  from  each  parent.  This  first  divi- 
sion of  the  egg  is  soon  followed  by  a  second,  which  produces 
four  cells,  and  in  this  division  the  same  process  is  repeated, 
the  chromatin  material  being  again  split  up  so  that  each  of 
the  four  cells  (Fig.  132  A)  contains  chromatin  material  from 
both  parents.  This  process  now  goes  on,  the  cells  dividing 
again  and  again,  until  the  original  egg  has  divided  into  a  large 
number  of  small  cells,  each  cell  probably  containing  chromatin 
material  from  both  parents.  This  process  of  segmentation  or 
cleavage  is  the  first  step  by  which  all  animals  and  plants  begin 
their  life  history,  the  egg  in  all  cases  dividing  after  a  similar 
manner  into  a  large  number  of  cells. 


282 


BIOLOGY 


FIG.  132.  —  DIAGRAM  REPRESENTING  THE  DEVELOPMENT  OF  THE  PROG 

A,  eight  stages  of  the  segmentation  of  the  egg;  B,  section  of  the  egg  showing  the  beginning 
of  the  differentiation  of  ectoderm  from  endoderm;  C,  sections  at  a  later  stage,  showing  the 
growth  of  the  ectoderm  over  the  endoderm;  D,  section  after  the  germ  layers  are  formed; 
«c,  ectoderm;  en,  endoderm;  mes,  mesoderm;  E,  surface  view  of  a  young  embryo  showing 
two  branchial  slits,  brc;  F,  surface  view  of  an  older  embryo;  G,  diagrammatic,  longitudinal 
sections  of  the  stage  F ;  H,  a  later  stage.  In  these  diagrams  the  ectoderm  is  in  black,  meso- 
derm, with  dotted  shading,  and  endoderm  without  shading. 

br,  the  brain;         I,  liver;  nc,  notocord;  sp,  spinal  cord. 

ht,  heart;  n,  nervous  system;         s,  sexual  duct; 

(Various  authors.) 


DEVELOPMENT  OF  THE  FERTILIZED  EGG  283 

2.  Differentiation. —  Although  the  cells  at  the  outset  are 
much  alike,  they  soon  begin  to  show  differentiation.     In  Fig- 
ure 132  B   it  will   be  seen  that  the   upper  cells  are  smaller 
than    the    lower    ones,   and   the   contents  of   the  larger  cells 
are  quite  different  from  those  of  the  smaller.    The  difference 
thus  shown  early  in  the  development  of  the  egg  marks  the 
distinction  between  those  cells  which  will  eventually  form  the 
alimentary  canal  and  those  which  will  form  the  other  parts 
of  the  body.    As  the  development  goes  on  and  the  number 
of  cells  in  the  embryo  increases  more  and  more,  greater  and 
greater  differences  are  found  among  them   (Figs.  C  and  D), 
so  that  one  group  of  cells  after  another  becomes  set  apart  by 
differences  in  structure  and  functions,  until  finally,  when  the 
animal  has  reached  the  adult  form,  it  is  not  only  composed  of 
innumerable  cells,  but  these  cells  have  assumed  a  great  variety 
of  shape  and  function.     This  process  of  gradual  change  of 
shape  and  function  of  cells  which  were  originally  alike,  is  spoken 
of  under  the  name  of  differentiation.    A  similar  change  occurs 
in  all  multicellular  animals  and  plants;  for,  after  segmentation 
of  the  egg,  there  always  follows  a  differentiation  of  cells. 

3.  The  Formation  of  Germ  Layers. —  After  the  cells  have 
multiplied  until  they  have  become  quite  numerous,  they  begin 
to  arrange  themselves  in  three  groups.    Soon  there  appears  an 
outer  layer,  an  inner  layer,  and  a -middle  layer,  known  respec- 
tively as  ectoderm,   endoderm,   and  mesoderm.     These  are 
shown  in  Figure  132  D,  which  represents  a  later  development 
in  the  frog.    The  method  by  which  these  three  layers  are  formed 
is  shown  diagrammatically  in  Figure  C.     It  may  briefly  be 
described  as  the  growing  of  the  mass  of  the  smaller,  ectoderm 
cells,  around  and  over  the  larger,  endoderm  cells,  so  as  finally 
to  bring  the  larger  cells  upon  the  inside  of  the  embryo,  surrounded 
by  the  smaller  ones.     Meantime  there   has  grown  from  the 
outer  and  inner  layers  a  third  mass  of  cells,  the  mesoderm, 
that  pushes  its  way  between  the  other  two,  thus  partly  filling 
up  the  space  between  the  outer  and  inner  layers;  Fig.  D.    The 


•284  BIOLOGY 

final  result  is  that  the  embryo  has  an  ectoderm  of  smaller  cells 
on  the  outer  side,  an  endoderm  of  larger  cells  on  the  inner  side, 
and  a  mesoderm  between  the  outer  and  the  inner  layer.  These 
three  layers  of  cells  remain  distinct,  and  are  destined  for  dif- 
ferent purposes  in  the  subsequent  life  of  the  animal,  each  one 
of  them  developing  into  certain  organs  only.  The  organs  that 
are  developed  from  the  three  layers  are  as  follows :  — 

The  mesoderm. —  From  the  mesoderm  develop  the  muscles, 
the  bones,  the  heart,  and  the  blood  vessels,  the  lining  of  the  body 
cavity,  the  outer  layer  of  the  alimentary  canal,  the  mesentery 
which  holds  the  alimentary  canal  in  position,  and  the  repro- 
ductive system. 

The  endoderm. —  From  the  endoderm  develop  the  alimentary 
canal,  the  glands  around  the  mouth,  the  lungs,  the  pancreas, 
and  the  liver.  The  muscles  which  form  the  wall  of  the  alimen- 
tary canal  are  developed  from  the  mesoderm,  but  the  lining  of 
the  digestive  tract,  with  all  its  glands,  which  secrete  the  digestive 
juices,  is  formed  from  the  endoderm. 

The  ectoderm. —  The  ectoderm  gives  rise  to  the  skin,  includ- 
ing the  epidermis  and  the  dermis.  It  also  grows  inward  to 
line  the  mouth  and  the  extreme  posterior  end  of  the  alimentary 
canal.  The  ectoderm  also  gives  rise  to  the  nervous  system, 
with  all  of  its  parts,  including  the  brain,  the  spinal  cord,  the 
nerves,  and  all  of  the  sensory  organs,  like  the  eyes,  the  ears, 
organs  of  smell  and  touch. 

It  will  be  seen  that  the  alimentary  canal  is  made  of  three 
parts:  the  anterior  end  is  formed  by  the  infolding  (imagination) 
of  the  ectoderm,  the  infolded  part  forming  the  mouth  or  buccal 
cavity;  the  posterior  end  is  also  formed  by  an  invagination  of 
the  ectoderm,  which  forms  the  cloacal  chamber;  the  rest  of  the 
canal  is  formed  from  the  endoderm.  These  three  parts  are 
called  the  foregut  (stomodceum) ,  the  midgut  (mesenteron) ,  and 
the  hindgut  (proctodceum) .  Similar  relations  are  found  in  other 
vertebrates  and  also  in  the  lower  animals  as  well. 

Layers  similar  to  those  described  are  found  in  the  embryos 


DEVELOPMENT  OF  THE  FERTILIZED  EGG  285 

of  nearly  all  animals.  Among  some  of  the  very  lowest  ( Hydra) 
only  the  ectoderm  and  the  endoderm  are  formed,  the  mesoderm 
being  omitted.  But  in  all  except  the  lowest  types  three  layers 
are  formed  early  in  the  embryological  history.  The  method 
by  which  these  three  layers  are  formed  differs  in  different  ani- 
mals. In  Figure  15  is  shown  a  method  of  formation  of  the 
endoderm,  differing  from  that  of  the  frog,  by  an  infolding  of  a 
hollow  sphere  to  form  a  double  sac.  But  however  differently 
the  layers  are  formed,  the  system  of  organs  which  are  developed 
from  them  is  essentially  the  same.  The  nervous  system  is 
always  developed  from  the  ectoderm,  the  alimentary  canal 
from  the  endoderm,  and  the  blood  system  and  muscles  are 
developed  from  the  mesoderm. 

4.  The  Formation  of  the  Body.—  While  the  germ  layers  have 
been  forming,  the  embryo  has  been  elongating  (Fig.  132  E\  and 
the  endoderm  forms  itself  into  a  hollow  tube  within  the  body, 
which  acquires  an  opening,  first  at  one  extremity  and  then  at 
the  other;  Fig.  G.  This  tube  becomes  the  alimentary  canal, 
and  the  two  openings  are  the  mouth  and  the  anal,  or  cloacal 
opening.  Between  this  inner  tube  and  the  outer  wall  of  the 
body  lies  a  cavity,  more  or  less  filled  with  the  mesoderm,  but 
in  it  eventually  appears  the  body  cavity  or  ccelum,  which  be- 
comes a  more  distinct  cavity  as  the  animal  grows.  Early  in 
the  development,  when  the  animal  has  assumed  the  form  shown 
at  E,  openings  in  the  side  of  the  neck  break  through  from  the 
alimentary  canal  to  the  exterior.  There  are  at  first  two  of  these, 
shown  at  E,  brc,  but  later  others  appear.  These  are  known 
as  branchial  openings,  and  become  passages  through  which 
water  taken  in  at  the  mouth  may  be  passed  to  the  exterior. 
They  represent  the  gill  slits  present  in  fishes,  and  are  to 
have  a  similar  function  a  little  later,  when  the  frog  hatches 
from  the  egg  and  lives  in  the  water.  While  these  changes  are 
going  on  there  is  formed  a  long,  thickened  rod  of  ectoderm  in 
the  middle  line  of  the  back,  extending  from  one  end  of  the  ani- 
mal to  the  other,  which  is  the  beginning  of  the  nervous  system; 


286  BIOLOGY 

Fig.  G,  n.  The  result  is  the  formation  of  a  little  animal  such 
as  is  shown  in  Figure  H,  in  which  the  relation  to  the  adult 
structure  can  be  clearly  seen,  although  at  this  stage  the  em- 
bryo only  slightly  resembles  the  adult  frog.  The  development 
that  has  taken  place  up  to  this  point  has  occupied  a  period  of 
several  days  from  the  time  when  the  egg  was  fertilized,  the 
exact  length  of  time  depending  to  a  large  extent  upon  the 
temperature,  the  different  stages  being  more  rapidly  passed 
through  if  the  eggs  are  kept  warm  than  when  they  are  kept  cool. 

Various  other  systems  of  organs  begin  to  appear  at  this  stage 
or  a  little  later.  From  the  ectoderm  along  the  middle  line  in 
the  back,  develops  a  rod  of  nervous  matter,  and  around  the 
front  end  of  this,  outgrowths  appear,  which  become  the  eyes, 
ears,  and  other  sense  organs.  The  nervous  mass  itself  differenti- 
ates into  the  brain  and  spinal  cord;  Fig.  H.  The  endodermal 
tube  also  develops  outgrowths  which  in  time  become  the  lungs, 
liver,  and  pancreas.  One  part  of  the  mesoderm  forms  itself 
into  a  gelatinous  rod  running  lengthwise  in  the  back  of  the 
embryo,  just  beneath  the  nervous  system.  This  is  the  noto- 
cord,  nc;  it  represents  the  beginning  of  the  spinal  column, 
and  in  time  the  vertebrce  grow  around  it.  Another  part  of  the 
mesoderm  develops  into  the  heart,  ht,  and  blood  vessels;  while 
that  part  of  it  which  lines  the  body  wall  becomes  the  muscles, 
and  that  which  is  next  to  the  intestine  develops  into  the  pm- 
toneum<smd  mesentery.  From  the  mesoderm,  too,  the  kidneys 
and  sexual  glands  arise,  with  their  ducts,  s. 

These  changes  take  place  quite  rapidly,  although  they  are 
not  completed  for  many  days.  When  they  are  finished  the 
whole  series  of  the  organs  of  the  frog  is  present,  though  yet 
incompletely  developed.  Meantime  the  animal  has  hatched 
from  the  egg,  and  forces  its  way  out  of  the  jelly  in  which  it 
has  been  embedded  and  assumes  an  independent  life. 

5.  Metamorphosis. —  The  further  development  of  the  frog 
comprises  a  number  of  different  stages,  shown  in  Figure  133, 
the  important  features  of  which  are  as  follows:  The  animal 


DEVELOPMENT  OF  THE  FERTILIZED  EGG  287 


B 


FIG.  133. —  THE  METAMORPHOSIS  OF  THE  FROG 

A,  the  embryo  within   the  egg;  C,  at  the  time  of  hatching.    At  about  the  stage  I,  t! 
animal  leaves  the  water  and  lives  a  part  of  the  time  in  the  air. 


288  BIOLOGY 

elongates,  and  a  slight  constriction  appears  behind  the  anterior 
end  resembling  a  neck.  The  front  portion  is,  however,  not 
the  head  alone,  but  the  head  and  body  fused  together,  while  the 
back  portion  soon  grows  out  into  an  elongated  tail.  From  the 
side  of  the  two  branchial  openings  feather-like  external  gills 
or  branchiae  develop,  which,  projecting  laterally  from  the  head, 
serve  as  respiratory  organs;  Fig.  D.  The  free  larva  is  now  known 
as  a  tadpole,  and  from  this  time  it  is  obliged  to  depend  upon 
itself.  Its  digestive  organs  have  become  developed  enough  to 
perform  their  functions,  and  the  larva  begins  to  feed  upon  vege- 
table food,  eating  the  delicate  green  plants  that  are  found  grow- 
ing on  the  bottom  of  the  pool  where  the  larva  attaches  itself. 
The  rapidity  with  which  the  animal  goes  through  the  subsequent 
changes  is  dependent  chiefly  upon  the  amount  of  food  it  obtains, 
and  the  temperature;  but  it  soon  begins  to  pass  through  the 
stages  represented  in  Figures  C  to  G.  The  front  end  of  the  body, 
which  is  the  head  and  body  fused  together,  increases  in  size 
and  becomes  rounded,  while  the  tail  elongates  and  becomes 
flatter,  serving  as  a  swimming  fin.  The  external  gills  disappear; 
but  the  gill  slits  remain,  the  animal  still  breathing  by  the  use 
of  internal  gills,  not  visible  from  the  outside.  The  size  of  the 
tadpole  varies  with  the  different  species  of  the  frog;  in  some  of 
the  ordinary  frogs  it  may  become  two  or  even  three  inches  in 
length,  while  in  other  species  it  is  not  more  than  half  an  inch. 

The  next  change  is  the  appearance  of  a  pair  of  small  pro- 
tuberances, or  buds,  on  the  posterior  end  of  the  body  on  either 
side;  Fig.  133  F.  These  grow  rapidly  in  length  and  develop 
into  the  hind  legs.  A  similar  pair  of  buds  appears  at  the  an- 
terior end  of  the  body,  a  little  behind  the  gill  slits,  which  later 
grow  into  the  fore  legs  or  arms.  As  these  legs  and  arms  grow, 
the  whole  shape  of  the  body  changes;  the  eyes  appear  on  the 
sides  of  the  head;  the  mouth,  which  is  at  first  a  round  sucking 
slit,  elongates  into  a  large  slit  surrounded  by  the  jaws;  the 
head  assumes  more  of  its  final  form;  the  shape  of  the  body 
changes  from  the  rounded  tadpole  to  a  more  elongated  structure. 


DEVELOPMENT  OF  THE  FERTILIZED  EGG  289 

The  tail  also  shortens  until  it  disappears.  It  does  not  drop  off, 
but  is  gradually  absorbed  into  the  blood  vessels  and  carried 
to  the  rest  of  the  body,  where  it  is  used  as  nourishment  for  the 
other  parts  of  the  body.  These  changes  are  not  abrupt  but  take 
place  gradually  as  the  animal  assumes  the  adult  form;  Fig. 
133,  F  to  K. 

By  the  time  the  form  shown  in  Figure  J  is  reached,  the  gill 
slits  have  entirely  closed,  the  skin  growing  over  them;  and  from 
this  time  on  the  animal  takes  air  into  its  mouth  and  forces  it 
into  its  lungs  in  the  ordinary  fashion  of  the  adult  frog.  It 
changes,  therefore,  from  a  water-breathing  to  an  air-breathing 
animal.  But  even  when  it  is  an  adult,  the  animal  never  quite 
loses  its  power  of  respiring  by  means  of  water,  for  the  skin 
of  the  adult  frog  is  always  kept  moist,  and  contains  abundant 
blood  vessels  by  means  of  which  oxygen  can  be  absorbed  from 
the  water,  and  carbon  dioxid  excreted.  Not  until  the  gill  slits 
have  closed  and  the  lungs  have  become  functional  is  the  frog 
able  to  leave  the  water  and  live  in  the  air.  By  this  time  its 
legs  have  become  well  grown  and  are  strong  enough  to  enable 
it  to  move  more  or  less  vigorously  on  the  land,  so  that  the 
tadpole  may  leave  the  water  and  assume  its  adult  habits. 

Other  Types  of  Metamorphosis. —  Such  a  series  of  changes 
from  the  embryo  to  the  adult  is  known  as  metamorphosis. 
Many  other  animals  besides  the  frog  have  a  metamorphosis. 
One  of  the  best-known  examples  is  the  metamorphosis  of  a 
butterfly,  which  hatches  as  a  caterpillar,  lives  a  considerable 
part  of  its  life  in  this  stage,  and  then  passes  into  a  pupa  iriside 
of  a  cocoon.  Here  it  remains  dormant  for  a  considerable  time 
and  eventually  emerges  in  the  form  of  a  winged  butterfly,  the 
imago.  Many  other  types  of  metamorphosis  are  found  among 
animals,  for  it  is  quite  common  for  them  to  pass  through  a 
series  of  stages  in  their  development,  each  stage  being  different 
from  the  other,  and  each  different  from  the  adult.  Not  all 
animals,  however,  have  a  metamorphosis,  many  passing  by  a 
very  direct  course  to  the  adult  stage.  In  the  ordinary  chick, 


290  BIOLOGY 

for  example,  the  embryo  pursues  the  most  direct  course  pos- 
sible for  building  itself  from  the  simple  egg  to  the  adult,  and 
the  chick,  when  it  hatches  from  the  egg,  is  practically  adult 
in  form,  although  not  in  size.  In  such  cases  we  call  the  history 
a  direct  development,  in  contrast  to  an  indirect  development 
or  a  metamorphosis. 

Embryology  a  Repetition  of  Past  History. —  It  will  be  seen 
from  the  development  of  the  frog  that  at  one  period  it  resembles 
a  fish  in  a  number  of  points.  It  lives  in  the  water,  has  a  flat 
swimming  tail,  possesses  branchial  slits,  and  carries  on  respira- 
tion by  means  of  gills.  The  study  of  geology  has  shown  that 
in  the  history  of  the  world  fishes  preceded  frogs,  and  it  is  thus 
seen  that  in  its  embryology  a  frog  shows  a  tendency  to  repeat 
the  past  history  of  animals.  Such  a  repetition  is  found,  not 
only  in  the  frog  but  in  many  other  animals,  for  it  is  a  funda- 
mental biological  law  that  embryology  repeats  past  history. 
In  technical  terms  this  is  expressed  by  the  statement  that  on- 
togeny is  a  repetition  of  phylogeny,  ontogeny  (Gr.  on  =  being  + 
-geneid)  being  the  individual's  embryological  history,  and  phy- 
logeny (Gr.  phylon  =  tribe  +  -geneia  =  producing)  the  history  of 
the  race,  during  the  geological  ages.  This  parallel  has  been  one 
of  the  strong  arguments  which  have  convinced  scientists  that  our 
present  forms  have  been  derived  by  ordinary  methods  of  de- 
scent, through  the  process  of  reproduction,  from  the  earlier  in- 
habitants of  the  world;  or,  in  other  words,  that  the  history  of 
the  organic  world  has  been  one  of  evolution  and  not  one  of  spe- 
cial- creation  of  each  species  independently,  as  was  formerly  be- 
lieved. While  a  few  years  ago  this  law  of  repetition  was  thought 
to  be  more  strictly  adhered  to  than  careful  study  has  proved  to 
be  the  case,  the  general  fact  that  embryology  tends  to  repeat 
past  history  remains  as  one  of  the  interesting  and  significant 
laws  of  nature.  It  is  sometimes  called  the  biogenetic  law. 

Oviparous  and  Viviparous  Animals. —  Many  animals  (for  ex- 
ample, the  frog)  extrude  their  eggs  into  the  water  as  soon  as  they 
are  mature  and  take  no  further  care  of  them.  In  some  cases, 


DEVELOPMENT  OF  THE  FERTILIZED  EGG  291 

as  in  birds,  snakes,  etc.,  the  eggs,  after  being  laid,  are  still  cared 
for  by  the  parents,  and  may  be  incubated  by  the  parents  to 
keep  them  warm  during  their  development.  All  animals  that 
thus  lay  eggs  are  called  oviparous  (Lat.  ovum  =  egg  -f-  parere 
=  to  bear).  A  few  of  the  higher  animals,  like  the  mammals, 
retain  the  egg  for  some  time  within  the  body  of  the  mother. 
The  sperms  from  the  male  in  these  animals  are  carried  into 
the  oviduct  at  copulation  by  the  penis,  and  the  eggs  are  fertil- 
ized while  they  are  still  within  the  oviduct.  After  the  egg  is 
fertilized  it  attaches  itself  to  the  part  of  the  oviduct  called  the 
uterus,  and  here  undergoes  development.  The  developing 
embryo,  called  the  foetus,  is  nourished  through  the  maternal 
blood  vessels,  and  grows  to  a  considerable  size  while  still  re- 
tained in  the  uterus  and  attached  to  it  by  a  membrane  called 
the  placenta.  Eventually,  when  it  has  become  mature,  it  is 
detached  from  the  uterus  and  expelled  to  the  exterior  at  birth. 
The  young  are  well  developed  at  birth,  and  such  animals  are 
spoken  of  as  viviparous  (Lat.  vivus  =  alive  +  parere  =  to  bear). 


CHAPTER  XV 

THE  SOURCE  AND  NATURE  OF  VITAL  ENERGY 
MATTER  AND  ENERGY 

PHYSICAL  science  teaches  that  the  universe  consists  of  twc* 
great  factors,  matter  and  energy. 

MATTER 

By  matter  is  meant  the  substance  of  the  objects  found  in 
nature,  such  as  earth,  stones,  etc.  One  of  the  fundamental  laws 
of  physics  is  that,  while  matter  may  be  changed  from  one  form 
to  another,  it  can  neither  be  created  nor  destroyed.  The  amount 
of  matter  in  the  universe  at  the  present  time  is  thus  exactly 
the  same  as  it  has  been  in  all  previous  ages. 

ENERGY 

By  energy  is  meant  the  force  or  power  that  exists  in  nature. 
Energy  is  the  power  of  doing  work,  and  may  best  be  explained 
by  illustrations. 

Active  Energy. —  A  cannon  ball  flying  through  the  air  is  said 
to  possess  energy.  It  is  flying  with  such  force  and  momentum 
that  it  requires  great  resistance  to  stop  it;  and  if  the  ball  could 
be  received  upon  properly  devised  machinery,  its  motion  might 
be  made  to  turn  wheels  or  do  any  other  kind  of  work.  The 
revolving  flywheel  of  an  engine  also  possesses  energy  of  the 
same  type,  its  motion  and  its  great  momentum  enabling  it,  if 
connected  with  machines,  to  move  them  and  make  them  do 
work.  In  the  same  way,  any  form  of  motion  is  energy.  In 
another  type  of  energy  the  motion  is  not  so  evident.  Heat, 
liberated  from  burning  coal,  is  energy,  since,  when  it  is  properly- 
applied  to  an  engine,  it  may  be  made  to  do  work.  In  this  case 
the  heat  may  be  applied  to  water,  which  it  vaporizes  into  steam, 
and  this  eventually  may  produce  motion  in  an  engine;  but  it 

292 


•THE  SOURCE  AND  NATURE  OF  VITAL  ENERGY        293 

is  fundamentally  the  power  in  the  heat  that  goes  into  the  engine 
and  finally  exhibits  itself  in  the  motion.  In  the  same  way,  the 
electric  current,  flashing  along  the  electric  wire,  is  energy, 
since  this  also,  if  received  by  a  proper  machine,  can  be  made 
to  set  machinery  in  motion  and  thus  accomplish  work.  Each 
of  these  four  examples  of  force  clearly  comes  under  the  defi- 
nition given,  since  they  all  show  the  power  of  doing  work. 
They  also  have  another  common  characteristic:  they  all  rep- 
resent motion.  The  cannon  ball  and  the  flywheel  are  evi- 
dently in  motion,  and  the  physicist  has  shown  that  heat 
and  electricity  .are  also  forms  of  motion.  Each  of  these  four 
examples,  then,  represents  energy  in  action.  An  indefinite 
number  of  other  examples  of  this  same  type  could  be  given, 
for  all  forms  of  light,  hoat,  motion,  chemical  action,  and  elec- 
tricity are  examples  of  energy,  and,  in  one  form  or  another,  all 
represent  energy  in  motion.  This  general  type  of  energy  in  mo- 
tion is  active  energy  or  kinetic  energy  (Gr.  kinetos  =  moving). 

Passive  or  Potential  Energy. —  Energy  is  not  always  active 
but,,  under  some  circumstances,  it  assumes  a  dormant  form, 
which  we  sped:  of  as  potential  energy.  By  the  term  "poten- 
tial" is  meant  that  the  energy,  though  not  at  the  moment 
active,  may  at  any  time  be  converted  into  active  energy.  Fcr 
example,  a  heavy  stone,  poised  on  the  roof  of  a  house,  is  at 
rest,  exhibiting  no  active  energy;  but  it  has  potential  energy, 
in  virtue  of  the  fact  that  it  is  raised  some  distance  above  the 
ground.  The  moment  it  is  dislodged  it  begins  to  move,  falling 
to  the  ground  by  the  law  of  gravitation,  and  as  it  falls  it  de- 
velops the  energy  of  motion.  No  energy  is  put  into  the  stone 
by  simply  dislodging  it  from  its  position  on  the  roof;  hence  it 
follows  that  the  stone  contained  the  energy  when  it  rested 
upon  the  roof,  only  the  energy  was  in  a  dormant  or  potential 
form.  When  it  was  dislodged  from  its  position  the  potential 
energy  began  to  be  active,  and  when  the  stone  reached  the 
earth  it  became  quiet  again,  its  energy  having  apparently 
disappeared. 


294  BIOLOGY 

A  different  type  of  potential  energy  is  illustrated  by  a 
bit  of  ordinary  coal.  The  coal  that  is  put  into  a  furnace 
contains,  stored  within  itself,  a  large  amount  of  energy  in  a 
dormant  form.  That  it  contains  the  energy  is  perfectly  evi- 
dent from  the  fact  that  we  need  only  put  it  under  proper  con- 
ditions, by  kindling  it,  and  the  energy  will  be  liberated  from 
the  coal  in  the  form  of  heat,  which  may  be  converted  into 
^notion  by  an  engine.  We  can  get  no  motion  out  of  the  steam 
engine  unless  we  put  the  energy  into  the  furnace  in  the  form 
of  coal,  wood,  or  other  fuel.  Evidently  fuels  may  be  looked 
upon  as  containing  a  store  of  dormant  energy.  These  types  of 
passive  energy,  which  exhibit  no  action,  but  which  are  capable 
of  being  brought  into  activity  when  placed  in  the  right  condi- 
tions, are  spoken  of  as  potential  energy  or  energy  of  position. 

THE  CONSERVATION  OF  ENERGY 

Energy  can  neither  be  created  nor  destroyed.  Just  as  we 
cannot  destroy  nor  create  matter,  so  we  cannot  destroy  nor 
create  energy,  the  amount  of  energy  present  in  our  universe 
to-day  being  the  same  as  it  has  been  in  all  previous  time.  This 
statement  does  not  seem  quite  so  self-evident  as  the  statement 
that  matter  cannot  be  created  or  destroyed,  for  many  ex- 
amples occur  that,  at  first  sight,  seem  to  be  instances  of  the 
destruction  of  energy.  A  stone  which  has  been  dislodged  from 
its  position  upon  the  roof  falls  rapidly  to  the  ground  and  de- 
velops energy  in  falling,  but  on  reaching  the  ground  it  stops 
suddenly  and  its  energy  seems  to  have  disappeared.  When  a 
cannon  ball  strikes  a  ledge  of  rock  it  suddenly  stops.  Any 
examples  of  the  stopping  of  motion  would  seem  to  be  illus- 
trations of  the  destruction  of  energy. 

A  careful  examination,  however,  shows  that  in  these  cases 
there  is  in  reality,  no  destruction  of  energy,  but  simply  the  con- 
version of  one  form  of  energy  into  another.  In  the  case  of  the 
stone  lodged  on  the  roof,  it  is  evident  that  at  one  time  a  cer- 
tain amount  of  energy  must  have  been  used  to  lift  this  stone 


THE  SOURCE  AND  NATURE  OF  VITAL  ENERGY        295 

into  its  position,  and  when  the  stone  fell  it  only  redeveloped 
.the  energy  that  was  originally  required  to  lift  it  to  its  position. 
The  amount  of  energy  required  to  lift  the  stone  to  its  position 
is  exactly  the  same  as  that  which  is  developed  by  the  stone 
when  it  falls  to  the  ground,  and  the  lifting  of  the  stone  and 
its  falling  illustrates  the  conversion  of  active  into  potential 
energy  and  reconversion  of  potential  energy  into  an  equivalent 
amount  of  active  energy.  It  would  seem,  however,  that  when 
the  stone  reaches  the  ground  the  energy  disappears.  But  if 
we  examine  the  fallen  stone  carefully,  and  the  earth  under- 
neath it,  we  find  that  both  have  been  warmed.  The  moment 
that  the  motion  of  the  stone  ceased,  heat  appeared.  Heat  is 
a  form  of  energy,  and  thus,  when  the  stone  comes  to  rest  on 
the  ground,  the  motion  of  the  stone  is  converted  into  that 
form  of  energy  which  is  called  heat.  This  heat  is  soon  dissi- 
pated from  the  stone  and  from  the  earth,  for  they  presently 
resume  their  former  temperature.  The  heat  has  simply  gone 
off  into  the  air;  it  is  not  destroyed  but  has  simply  distributed 
itself,  and  slightly  raised  the  temperature  of  the  air.  Nowhere 
in  this  series  of  changes  has  there  been  any  loss  of  energy,  but 
simply  the  conversion  of  one  form  into  another.  Some  5000 
years  ago  the  Egyptians  lifted  a  large  number  of  stones  and 
placed  them  one  on  top  of  another  so  as  to  make  the  pyramids, 
exerting  a  large  amount  of  energy;  the  energy  used  in  placing 
the  stones  in  position  was  stored  away  in  the  pyramids  in  the 
form  of  potential  energy  and  is  there  still.  If  at  any  time  the 
pyramids  should  topple  over  and  the  stones  fall  to  the  ground, 
there  would  be  redeveloped  an  amount  of  motion  exactly  equal 
to  the  amount  used  to  lift  them  in  position.  Thus  energy 
may  be  stored  away  and  remain  in  a  potential  form  for  ages; 
but  at  any  future  time  the  energy  originally  stored  away  may 
reappear  in  the  form  of  active  energy. 

The  energy  present  in  a  dormant  form  in  coal  requires  a 
little  more  explanation.  Chemists  have  shown  that  the  small- 
est particles  of  matter  which  we  can  see  are  themselves  made  of 


296  BIOLOGY 

much  smaller  particles  called  atoms,  which  are  quite  invisible 
even  with  the  highest-power  microscope.  They  also  tell  us 
that  these  atoms  are  united  in  groups,  which  are  called  mole- 
cules, each  consisting  of  a  number  of  atoms.  Just  as  it  re- 
quires the  expenditure  of  energy  to  lift  stones  into  position 
to  form  a  monument,  it  also  requires  energy  to  lift  atoms 
into  position  to  form  a  molecule;  and  if  the  molecule  is 
broken  down,  the  energy  is  liberated  according  to  the  same 
principle  concerned  in  liberating  it  when  a  monument  falls. 
If,  therefore,  we  look  upon  the  particle  of  coal  as  a  series  of 
molecules,  each  built  up  of  many  atoms,  it  follows  that  if  these 
tiny  molecules  are  broken  down,  so  that  their  atoms  will  assume 
a  simpler  form,  the  energy  imprisoned  in  them,  in  a  dormant 
state,  will  be  released.  Coal  is  thus  made  of  immense  numbers 
of  complex  molecules,  each  of  which  has  been  built  by  the 
expenditure  of  energy,  and  the  coal  contains,  in  a  potential 
form,  energy  which  may  be  released  by  breaking  up  the  coal. 
The  molecule  is  broken  down  when  the  coal  is  burned  and  its 
energy  appears  in  the  form  of  heat,  which  may  then  be  applied 
to  the  moving  of  an  engine.  This  of  course  raises  the  question 
as  to  how  the  energy  was  stored  away  in  the  coal, —  a  question 
to  which  we  will  refer  later. 

THE  TRANSFORMATION  OF  ENERGY 

Any  type  of  energy  may  be  converted  into  any  other  type. 
When  we  lift  a  stone  to  the  roof  of  the  house  we  convert  energy 
of  motion  into  energy  of  position,  and  when  the  stone  falls, 
energy  of  position  is  converted  again  into  energy  of  motion. 
When  it  is  halfway  to  the  ground,  it  has  a  certain  amount  of 
energy  of  motion,  because  it  is  moving;  but  it  also  has  a  cer- 
tain amount  of  energy  of  position,  because  it  is  considerably 
above  the  surface  of  the  earth.  The  more  closely  it  approaches 
the  earth,  however,  the  more  its  energy  of  position  is  converted 
into  energy  of  motion,  and  the  moment  it  strikes  the  ground, 
all  of  its  energy  of  motion  is  converted  into  heat.  The  potential 


THE  SOURCE  AND  NATURE  OF  VITAL  ENERGY        297 

energy  in  the  coal  in  the  furnace  is  converted  into  heat ;  the  heat 
is  converted  by  the  engine  into  motion;  the  motion  of  the  fly- 
wheel, by  being  attached  to  a  dynamo,  may  be  converted  into 
electricity;  the  electricity,  passing  over  the  wires,  may  run 
into  an  electric  lamp,  where  it  is  converted  into  light,  or  it 
may  go  into  an  electric  stove  to  be  converted  into  heat.  The 
motion  of  water  over  a  waterfall  may  easily  be  converted  into 
the  motion  of  a  wheel  by  the  means  of  a  water-wheel,  this  into 
electricity,  and  this  in  turn  into  light,  heat,  motion,  or  any  other 
form  of  energy  that  we  wish  to  obtain. 

Some  of  the  types  of  transformation  of  energy  are  more  easy  to 
bring  about  than  others.  It  is  much  easier  to  convert  motion 
into  heat  than  to  convert  heat  into  motion.  Any  form  of  mo- 
tion is  sure  to  take  the  form  of  heat  eventually,  whether  we 
are  turning  a  grindstone  or  putting  a  brake  on  a  railroad  train, 
or  whether  a  cannon  ball  is  stopped  by  a  stone  cliff.  Heat, 
indeed,  seems  to  be  the  type  which  all  forms  of  energy  have  a 
tendency  to  assume  in  the  end;  it  is  then  radiated  into  the  atmos- 
phere and  into  space,  where  it  is  beyond  the  reach  of  this  earth 
and  is  called  radiant  heat.  It  is  true  that  we  have  some  devices 
by  which  heat  may  be  reconverted  into  motion,  but  always  with 
considerable  loss  as  radiant  heat.  We  put  into  our  steam  en- 
gines five  times  as  much  stored  energy  in  the  form  of  coal  as 
we  receive  in  return  in  the  form  of  motion,  not  because  the 
energy  is  destroyed,  but  because  four-fifths  of  the  energy  of 
the  coal  is  wasted  in  heating  the  machinery  and  the  air,  and 
then  passes  away  as  radiant  heat,  only  a  small  part  being  con- 
verted into  motion. 

Definition  of  a  Machine. —  A  machine  is  any  device  which 
converts  one  form  of  energy  into  another.  The  locomotive  is 
a  machine  for  converting  heat  into  motion;  the  electric  bulb 
is  a  machine  for  converting  electricity  into  light;  the  motor 
converts  electricity  into  motion.  Even  the  gas  burner  is  a 
machine  for  converting  the  chemical  energy  of  the  gas  into 
light.  A  clock  is  a  machine  which  converts  the  potential 


298  BIOLOGY 

energy  in  its  coiled  spring  into  the  motion  of  its  pendulum 
and  hands;  a  sailboat  is  a  machine  for  converting  the  energy 
of  the  wind  into  the  motion  of  the  boat.  So  one  might  illus- 
trate indefinitely.  In  no  case  is  there  any  creation  of  energy 
by  the  machine,  simply  the  conversion  of  one  form  into  an- 
other. Not  only  is  there  no  creation  of  energy,  but  there  is 
an  actual  loss  of  available  energy,  inasmuch  as  heat  always 
develops,  and  after  energy  has  assumed  the  form  of  heat,  as 
we  have  just  seen,  it  is  difficult  to  get  it  back  into  another 
form.  While  there  is  no  actual  destruction  of  energy  when  it 
is  converted  into  heat,  there  is,  in  every  form  of  machinery 
with  which  we  are  acquainted,  a  loss  of  available  energy.  Some- 
times this  loss  is  very  great.  For  example,  in  an  ordinary 
electric  lamp  about  95%  of  the  electrical  energy  that  is  put 
into  the  bulb  is  lost;  only  5%  of  it  appears  as  light.  The  effi- 
ciency of  a  machine  is  indicated  by  the  percentage  of  the  energy 
supplied  which  we  can  get  back  in  the  form  that  we  desire. 
Machines  differ  much  in  their  efficiency  in  this  respect.  It  is 
quite  easy  to  get  very  efficient  machines  for  converting  motion 
into  heat,  but  very  difficult  to  get  an  efficient  machine  for  con- 
verting heat  into  motion.  The  most  efficient  machines  that 
we  have  for  this  latter  purpose  are  gas  engines,  some  of 
which  give  back  25%  or  30%  of  the  energy  put  into  them. 
Most  engines  give  a  far  smaller  proportion  than  this.  Many 
steam  engines  give  back  as  motion  not  more  than  5%  to  10% 
of  the  energy  furnished.  This  matter  of  efficiency  is  one  of 
interest  as  we  come  to  study  the  power  of  living  organisms  to 
convert  one  type  of  energy  into  another. 

THE  LIVING  ORGANISM  AS  A  MACHINE 

From  the  definition  above  given  it  is  very  easy  to  see  that 
the  living  organism,  either  animal  or  plant,  is  a  machine,  since 
it  is  a  mechanism  which  transforms  one  type  of  energy  into 
another.  This  may  best  be  understood  by  considering  first 
the  life  of  plants  and  then  that  of  animals. 


THE  SOURCE  AND  NATURE  OF  VITAL  ENERGY        299 

THE  LIFE  OF  A  PLANT 

Sunlight  furnishes  the  earth  with  practically  all  its  energy. 
There  have  been  many  attempts  to  make  efficient  sun  engines, 
which  will  utilize  the  rays  of  the  sun  to  serve  directly  as  a 
source  of  energy  sufficient  to  run  engines.  Sun  engines  have 
been  made,  but  as  yet  they  are  cumbersome,  unwieldy,  and  im- 
practical. But  it  seems  that  the  time  must  come,  after  the  ex- 
haustion of  the  coal  supply,  when  sun  engines  will  be  a  necessity. 
A  plant  growing  on  the  surface  of  the  earth  is  a  perfectly  efficient 
sun  engine,  devised  by  nature  to  utilize  the  rays  of  the  sun 
and  then  to  transfer  the  energy  thus  received  to  the  rest  of 
the  living  world.  The  life  of  the  ordinary  green  plants  consists 
of  two  features:  (1)  the  utilization  of  the  sun's  rays  and  the 
storing  away  of  these  rays  in  a  form  of  potential  energy; 
(2)  the  liberation  of  this  energy  and  its  subsequent  use  by 
the  plant.  These  two  processes  will  be  considered  in  turn. 

Energy  Stored  by  Plants. —  All  green  plants  have  the  power 
of  absorbing  the  sun's  rays  and,  by  the  means  of  energy  thus 
obtained,  of  building  up  chemical  compounds  of  great  complex- 
ity which  will  contain  the  energy  thus  absorbed,  stored  away  in 
a  potential  form.  Their  method  of  accomplishing  this  is  in  part 
as  follows:  In  Chapter  VI  we  have  learned  that  plants  have 
the  power  of  manufacturing  starch  out  of  carbon  dioxid  and 
water.  This  process  involves  the  manufacture  of  complex 
molecules  (C6Hi0O5)  out  of  simple  ones  (H2O  and  CO2),  and 
hence  requires  the  expenditure  of  energy.  Since  it  can  take 
place  only  in  sunlight,  it  becomes  evident  that  (1)  the  sun's 
rays  are  the  source  of  energy  used,  that  (2)  the  starch  manu- 
factured will  contain  in  a  potential  form  the  energy  used  in 
building  it,  and  that  (3)  this  energy  may  be  liberated  in  an 
active  form  if  the  starch  molecule  is  broken  down. 

Stored  Energy  Utilized  by  Plants. —  The  energy  stored  in 
the  starch  is  the  primary  source  of  energy  for  nearly  all  the 
activities  on  the  earth,  except  water  power.  The  plant  uses  it 
for  two  distinct  purposes:  L  While  plants  do  not  in  their 


300  BIOLOGY 

ordinary  life  exhibit  a  great  amount  of  active  energy,  they  do 
develop  a  little  heat  and  a  little  motion,  and  they  are  constantly 
lifting  quantities  of  water  from  the  soil  to  the  tops  of  the 
branches.  All  this  requires  energy,  which  is  obtained  by  break- 
ing down  some  of  the  starch  and  utilizing  the  energy  thus  lib- 
erated. 2.  Plants  are  always  at  work  building  other  materials 
besides  starch.  Proteids,  woods,  and  fats  are  manufactured  by 
combining,  within  the  living  cells,  the  various  materials  ab- 
sorbed by  the  roots  (nitrates,  etc.),  with  the  starches  made  in 
the  leaves.  The  chemical  processes  by  which  these  new  organic 
compounds  are  built  are  not  yet  understood,  but  one  feature 
is  significant.  Just  as  starches  are  more  complex  than  the 
water  and  carbon  dioxid  out  of  which  they  are  made,  so  the 
proteids  are  far  more  complex  than  the  starches,  nitrates,  etc., 
out  of  which  they  are  made.  Since  it  requires  energy  to  build 
the  complex  molecule  starch  out  of  the  simpler  carbon  dioxid 
and  water,  so  too  it  requires  energy  to  build  proteids  out  of 
the  starches  and  nitrates.  For  this  purpose  the  plants  do  not 
use  the  sun's  rays  directly,  but  they  use  the  energy  they  have 
stored  in  the  starch.  In  other  words,  in  making  proteids,  a  cer- 
tain quantity  of  starch  or  sugar  is  broken  down  into  a  condition 
of  carbon  dioxid  and  water,  and  as  a  result  of  this  destruction 
the  stored  energy  in  the  sugar  molecule  is  liberated.  This 
energy  is  liberated  within  the  living  cells,  and  under  such 
conditions  the  protoplasm  can  make  use  of  it  for  building  the 
complex  proteids  out  of  the  simpler  materials.  This  general 
process  is  called  metastasis. 

Thus  it  is  seen  that  the  plant  protoplasm  uses  the  starches 
for  a  double  purpose.  Part  of  them  are  reduced  to  the  condi- 
tion of  carbon  dioxid  and  water  in  order  to  liberate  the  energy 
needed  by  the  plant.  Part  of  them  are  combined  with  other 
ingredients  to  enter  into  the  combination  of  proteids,  etc.  By 
this  latter  process  there  is  thus  (1)  an  accumulation  of  proteids 
and  other  substances  in  the  plant  body,  (2)  a  destruction  of 
sugar  or  starchr  (3)  an  elimination  of  carbon  dioxid  and  water, 


THE  SOURCE  AND  NATURE  OF  VITAL  ENERGY        301 

arising  from  the  destruction  of  that  portion  of  the  starch  which 
was  utilized  as  a  source  of  energy  for  the  constructive  processes. 
The  carbon  dioxid  and  water  are  waste  products  and  are  liber- 
ated at  once  by  the  process  of  excretion. 

Thus  it  will  be  seen  that  there  are  two  processes  going  on 
in  a  plant  body.  One  —  photosynthesis  —  is  a  constructive 
process  by  which  the  sun's  energy  is  stored;  the  other — metas- 
tasis —  is  a  destructive  process  by  which  the  energy  is  liber- 
ated. The  former  process  is  going  on  in  green  leaves  and  only 
in  sunlight;  the  latter  takes  place  in  all  of  the  living  parts  of 
the  plant,  whether  in  sunlight  or  in  darkness,  at  all  times  when 
the  plant  is  carrying  on  its  life  processes.  By  the  former 
process  starch  is  being  made;  by  the  latter  the  plant  manu- 
factures a  host  of  materials  which  are  stored  away  in  its  body 
in  the  form  of  proteids,  wood,  fat,  cellulose,  or  other  substances. 

Plants  Produce  an  Excess  of  Organic  Material. —  In  all  gre^n 
plants,  photosynthesis  is  much  in  excess  of  the  metastasis,  and 
green  plants  are  constantly  manufacturing  a  quantity  of  starch 
and  other  organic  products,  far  more  than  they  need  for  their 
own  use. 

The  materials  thus  produced  serve  not  only  as  a  reserve 
for  their  own  future  use  but  also  for  most  other  forms  of  ac- 
tivity on  the  earth.  All  fuel  which  is  used  by  our  numerous 
engines,  whether  wood,  coal,  oil,  or  gas,  can  be  traced  back  to 
plant  life,  and  represents,  therefore,  the  sun's  energy  stored 
by  photosynthesis.  The  food  of  all  animals  also  comes  from 
plants. 

THE  LIFE  OF  AN  ANIMAL 

Stored  Energy  Utilized  by  Animals. — The  only  source  of 
energy  available  for  animals  and  colorless  plants  is  that  stored 
up  by  green  plants,  and  rendered  available  when  liberated  by 
the  destruction  of  the  compounds  that  hold  it.  The  general 
result  of  animal  life  is  a  destructive  one,  with  its  resulting 
liberation  of  potential  energy.  Animal  protoplasm  is,  however, 
able  to  carry  on  some  constructive  work.  It  can  make  fats 


302  BIOLOGY 

out  of  starches,  can  convert  one  proteid  into  another,  and  can 
make  new  living  protoplasm  if  fed  with  lifeless  proteids;  all  of 
these  are  constructive  processes.  Whatever  energy  is  needed 
for  this  work  must  be  obtained  by  breaking  down  part  of  the 
food,  so  that  the  result  is  a  reduction  of  the  total  amount  of 
organic  materials.  In  their  constructive  work,  animals  are  not 
only  unable  to  make  starches  and  sugars,  but  they  are  unable 
to  make  proteids.  Since  they  require  these  as  materials  out 
of  which  to  manufacture  muscles,  nerves,  glands,  etc.,  it  fol- 
lows that  they  are  dependent  upon  plants,  not  only  for  starches 
but  also  for  proteids,  which  the  plants  manufacture  and  which 
the  animals  utilize. 

From  this  outline  of  the  transformation  of  energy  it  is  evi- 
dent that  living  organisms,  both  animals  and  plants,  are  in 
a  strict  sense  machines.  That  living  beings  possess  special 
powers  shown  by  no  other  kind  of  mechanism,  and  therefore 
belong  in  a  category  by  themselves,  is  very  evident;  but  so  far 
as  concerns  the  problem  of  energy  they  are  machines.  Vital 
energy  is  only  the  energy  of  sunlight  transformed  into  various 
types  within  the  mechanism  of  the  living  machine.  Since 
coal  is  simply  an  accumulation  of  the  remains  of  plant  life  of 
past  ages,  we  now  see  the  source  of  its  energy.  It  contains 
the  stored  sunlight  of  the  past. 


CHAPTER  XVI 
THE  MECHANICS  OF  THE  LIVING  MACHINE 

IN  the  general  comparison,  of  the  living  body  with  a  machine, 
a  number  of  significant  conclusions  are  reached  when  we  carry 
this  comparison  out  in  detail. 

Are  the  Income  and  Outgo  Equivalent? — Can  all  of  the  en- 
ergy shown  by  the  living  organism  be  accounted  for  by  the  energy 
furnished  by  the  food,  and,  conversely,  can  all  of  the  energy  fur- 
nished in  the  food  be  accounted  for  in  the  form  of  energy  exhib- 
ited in  the  living  organism? 

If  the  law  of  the  conservation  of  energy  is  correct,  the 
answers  to  these  questions  must  be  in  the  affirmative.  To  get 
an  experimental  answer  is  not  easy,  but  it  has  been  done,  as 
follows:  A  large  box  has  been  constructed  in  which  is  placed 
an  animal,  or  sometimes  a  human  being,  and  then  the  box  is 
sealed.  By  means  of  ingenious  apparatus  the  person  inside  of 
the  box  is  furnished  with  the  necessary  air  to  carry  on  his 
respiration,  and  is  given  plenty  of  food  and  water;  he  remains 
in  this  box  for  a  varying  length  of  time.  The  apparatus  is 
designed,  not  only  to  determine  the  exact  amount  of  water 
and  food  that  the  individual  consumes,  but  also  the  amount  of 
oxygen  he  takes  from  the  air,  the  carbon  dioxid  he  breathes 
into  the  air,  together  with  all  the  moisture  that  is  eliminated 
from  the  body,  and  all  other  excretions.  Moreover,  the  amount 
of  energy  furnished  him  in  his  food  is  measured,  and  the  amount 
of  heat  liberated  from  his  body  is  determined  with  accuracy, 
as  well  as  the  amount  of  work  that  he  does. 

If  the  doctrine  of  conservation  of  energy  holds  concerning 
the  animal  body,  as  it  does  concerning  other  machines,  it  ought 
to  be  found  by  such  an  experiment  that  the  amount  of  energy 
exhibited  by  the  individual  is  identical  with  that  furnished 
in  his  food,  and  that  the  amount  of  excretions  is  exactly  equiv- 

303 


304  BIOLOGY 

alent  to  the  amount  of  food  consumed  in  his  body  during 
this  given  time.  The  difficulties  of  carrying  on  such  an  experi- 
ment have  been  great,  but  they  have  been  surmounted  satis- 
factorily, and  the  results  are  always  the  same.  There  is  an 
exact  equivalence  between  the  income  and  the  outgo  of  a  liv- 
ing animal,  both  as  to  force  and  matter.  The  amount  of 
excretion  from  the  individual  is  exactly  equal  to  the  amount 
of  food  consumed;  and  the  amount  of  energy  developed  is 
the  exact  equivalent  of  the  energy  contained  in  the.  food  that 
he  uses  during  the  same  experiment.  The  general  conclusion 
is  that  the  income  and  the  outgo  of  an  animal  balance,  and  that 
the  living  machine,  like  other  machines,  simply  transforms  one 
form  of  energy  into  another  without  creating  or  destroying  it. 
In  this  statement  no  account  is  made  of  the  energy  of  the 
action  of  the  nervous  system,  which  does  not  show  itself  in 
such  experiments,  the  probable  reason  being  that  the  record- 
ing apparatus  is  too  coarse  to  show  an  amount  of  energy  so 
slight  as  that  exhibited  by  the  nervous  svstem. 

DETAILS  OF  THE  ACTION   OF  THE    MACHINE 

In  the  running  of  an  ordinary  machine,  like  a  steam  engine, 
we  understand  fairly  well  the  details  of  its  action.  We  can 
understand  how  the  forces  of  chemical  affinity  break  up  the 
chemical  compounds  in  coal;  how  the  heat  thus  liberated 
vaporizes  the  water;  how  the  water  under  pressure  acts  on  the 
piston  in  the  cylinder,  and  how  this  produces  the  revolution 
of  the  flywheel.  It  is  true  that  we  do  not  understand  the 
forces  of  chemical  affinity  by  which  coal  burns,  but,  apart  from 
this,  there  is  nothing  mysterious  in  the  fact  that  the  engine 
converts  the  stored-up  energy  contained  in  the  coal  into  the 
motion  of  the  flywheel.  Is  a  similar  intelligible  explanation 
possible  of  the  activities  that  go  on  in  the  living  organism? 
In  other  words,  do  chemical  and  physical  forces  suffice  to  ex- 
plain the  activity  of  the  living  machine,  just  as  they  do  the 
activity  of  the  non-living  machine? 


MECHANICS  OF  THE  LIVING  MACHINE  305 

To  follow  out  this  question 'in  detail  would  take  more  space 
than  could  be  devoted  to  it  here.  A  few  of  the  more  important 
functions  of  life  may  be  considered,  and  will  serve  to  show 
how  modern  biological  science  endeavors  to  explain  life  phe- 
nomena in  terms  of  chemical  and  physical  forces.  In  this 
discussion  we  shall  confine  our  attention  wholly  to  the  life  of 
animals.  The  life  of  plants  is  far  simpler  than  that  of  animals, 
and  if  it  can  be  shown  that  the  animal  organism  works  in  a 
mechanical  fashion,  we  may  safely  assume  that  the  same  prin- 
ciple will  hold  for  the  vegetable  kingdom.  In  following  out 
this  thought  we  will  consider  in  succession  several  of  the  im- 
portant functions  of  animal  life. 

Digestion. — Digestion  is  simply  a  chemical  change  in  the 
nature  of  the  food,  and  involves  nothing  mysterious,  nor  any 
special  forces.  The  foods  when  taken  into  the  body  are  mostly 
insoluble.  In  order  to  pass  through  the  walls  of  the  intestine, 
they  must  first  be  dissolved  in  the  liquids  of  ^the  digestive 
tract,  and  before  they  are  dissolved  they  must  be  changed  into 
a  soluble  condition.  The  changes  which  make  them  soluble 
are  not  peculiar  to  the  living  body,  since  they  will  take  place 
equally  well  in  a  chemist's  laboratory.  One  of  the  most  impor- 
tant steps  in  digestion  is  the  change  of  starch  into  sugar;  and 
starch,  by  proper  chemical  methods,  can  be  changed  into  sugar 
just  as  readily  in  the  test  tube  of  a  laboratory  as  in  the  digestive 
organs  of  an  animal.  The  digestion  of  starch  has  nothing  mys- 
terious in  it,  and  is  only  an  instance  of  the  application  of  the  well- 
known  chemical  forces.  The  same  thing  is  true  of  all  the  other 
changes  in  the  food  which  we  call  digestion.  They  are  all 
chemical  changes,  resulting  from  the  laws  of  chemical  affinity. 
The  only  feature  concerning  the  process  that  is  not  intelligible 
in  terms  of  chemical  law  is  the  nature  of  the  digestive  juices. 
The  digestive  juices  contain  substances  that  have  the  power 
to  bring  about  chemical  changes.  If  we  mix  starch  and  water 
together  they  will  not  combine  to  make  sugar,  but  will  remain 
a  mixture  of  starch  and  water.  If,  however,  to  this  mixture 


306  BIOLOGY 

we  add  a  little  of  the  secretion  of  the  pancreas,  the  starch  and 
the  water  will  chemically  combine  to  produce  sugar,  a  new 
compound.  The  pancreas  produces  a  substance  which  is  called 
amylopsin,  which  has  the  power  of  causing  a  chemical  union 
of  the  starch  with  the  water.  This  substance  we  call  an  enzyme. 
It  is  not  alive  nor  does  it  need  any  living  environment  for  its 
action.  If  we  separate  a  little  of  it  from  the  pancreatic  juice 
and  put  it  in  a  test  tube  with  water  and  starch,  it  will  cause 
the  union  of  the  water  and  the  starch  exactly  as  it  does  in  the 
digestive  tract.  Now  we  do  not  know  exactly  the  nature  of  this 
enzyme,  nor  just  how  it  brings  this  union  about;  therefore  the 
vital  process  of  digestion  is  not  entirely  understood  at  present. 
We  do  know,  however,  that  digestion  itself  is  only  a  chemical 
change,  and  that  the  same  chemical  union  of  the  starch  with 
the  water  can  be  brought  about  without  the  presence  of  this 
enzyme.  The  fact  that  we  do  not  exactly  understand  how 
the  pancreatic  juice  acts  in  this  case  is  no  stranger  than  the 
fact  that  we  do  not  understand  exactly  how  a  spark  causes  a 
bit  of  gunpowder  to  explode.  We  do  not  doubt  that  the  ex- 
plosion of  the  powder  is  the  result  of  chemical  and  physical 
forces,  and  there  is  no  more  reason  to  doubt  that  the  combina- 
tion of  the  starch  with  the  water,  under  the  influence  of  amy- 
lopsin, is  also  the  result  of  chemical  and  physical  forces. 

The  same  principle  holds  in  regard  to  the  digestion  of  all 
other  foods  in  the  digestive  tract  of  animals.  Each  of  the  di- 
gestive juices  contains  special  enzymes,  each  food  is  acted 
upon  by  enzymes,  and  in  all  cases  the  food  undergoes  a  chemi- 
cal change.  Apart  from  the  fact  that  they  are  brought  about 
by  these  enzymes,  there  is  little  or  nothing  to  distinguish  be- 
tween chemical  changes  taking  place  in  the  body  and  similar 
changes  taking  place  outside  of  the  body.  Digestion,  in  other 
words,  is  a  chemical  process  and  controlled  by  chemical  laws. 

The  Absorption  of  Food. —  The  digested  food  passes  through 
the  intestine,  being  forced  along  by  the  muscular  action  of 
the  intestinal  wall.  As  it  passes  through  the  intestine  it  is 


THE  MECHANICS  OF  THE  LIVING  MACHINE 


307 


gradually  absorbed,  soaking  through  into  the  blood  vessels 
that  lie  within  the  walls.  This  process  of  food  absorption  in- 
volves another  set  of  forces,  which  are,  at  least  to  a  considerable 
extent,  either  chemical  or  physical.  The  primary  force  con- 
cerned is  what  physicists  call  osmosis  or  dialysis,  a 
force  which  has  no  special  connection  with  life.  If 
a  membrane  separates  two  liquids  of  different  con- 
sistency (Fig.  134),  a  force  is  exerted  on  the  liquids 
that  causes  each  to  pass  through  the  membrane  in 
an  opposite  direction,  until  the  constitution  of  the 
liquids  on  the  two  sides  of  the  membrane  is  the 
same.  The  force  that  drives  these  liquids  through 
the  membrane  is  a  powerful  one,  since  it  is  exerted 
against  a  high  pressure.  In  Figure  134  a  mem- 
branous bladder  is  attached  to  the  lower  end  of  a 
glass  tube.  If  a  solution  of  sugar  is  placed  inside 
of  this  bladder  and  pure  water  outside  of  it,  the 
sugar  and  the  water  will  both  pass  through  the 
membrane  in  opposite  directions.  Under  these  cir- 
cumstances, however,  more  water  passes  from  the 
outside  into  the  bladder  than  passes  from  the  blad- 
der outward.  The  result  is  that  the  bladder  be- 
comes more  and  more  filled  with  liquid,  and  enough 
pressure  is  produced  in  the  bladder  to  force  the 
water  up  the  tube,  in  which  it  may  rise  to  quite  a 
height.  This  force  is  known  as  osmosis,  and  it  is 
always  exerted  whenever  two  solutions  of  unequal 
concentration  are  separated  from  each  other  by  a 
membrane.  Some  substances,  like  the  white  of  an 
egg,  are  not  capable  of  passing  through  a  membrane, 
and  we  refer  to  them  by  the  term  colloidal  or  non- 
dialyzable.  Other  substances,  like  salt  and  sugar,  will  readily 
pass  through  membranes,  and  we  speak  of  them  as  crystalline 
or  dialyzable. 

Osmosis  is  the  fundamental  force  concerned  in  the  absorption 


FIG.  134.— 
A  DIAGRAM 

ILLUSTRAT- 
ING  THE 
FORCE  OF 
OSMOSIS 


308 


BIOLOGY 


of  the  food  from  the  alimentary  canal.  Undigested  foods  are 
not,  as  a  rule,  capable  of  osmosis.  Digestion  changes  them 
into  a  condition  in  which  they  are  soluble  and  capable  of  os- 
mosis. After  complete  digestion  the  foods  in  the  alimentary 
canal  have  been  converted  into  a  dialyzable  liquid.  More- 
over, the  structure  of  the  intestine  is  such  as  to  make  osmosis 
a  natural  process.  This  can  be  under- 
stood from  Figure  135,  which  illustrates 
a  diagrammatic  cross  section  of  the  in- 
testinal wall.  In  such  a  figure  the  food 
occupies  the  space,  in.  The  walls  of  the 
intestine  are  thrown  into  little  papillae 
called  villi,  each  of  which  is  covered  by  a 
membrane,  m;  on  the  other  side  of  this 
membrane,  at  bv,  there  are  blood  vessels 
containing  the  blood,  which  is  a  liquid 
of  very  different  nature  from  the  intes- 
tinal contents.  Thus  it  is  seen  that  we 
have  a  membrane  separating  two  liquids 
of  different  consistency,  the  blood  on 
the  one  side  and  the  digestive  food  on 
the  other.  Under  these  circumstances, 
the  force  of  osmosis  will  develop  and  the 
material  in  the  solution  will  begin  at 
once  to  pass  through  the  membrane 
from  one  side  to  the  other.  Thus  the  primary  factor  in  the 
absorption  of  food  from  the  intestines  is  that  of  osmosis. 

The  physical  force  of  osmosis  is  not,  however,  the  only  factor 
concerned  in  the  absorption  of  food.  If  it  were,  there  would 
be  an  equivalent  passage  of  liquid  from  the  blood  into  the 
intestine,  as  well  as  from  the  intestine  into  the  blood.  Such  an 
equivalent  passage  from  the  intestine  does  not  seem  to  take 
place,  proving  that  the  forces  concerned  in  the  absorption  of  food 
are  not  confined  to  the  process  of  osmosis.  Moreover,  a  careful 
study  of  the  absorptive  process  shows  that  it  is  much  more 


FIG.  135. —DIAGRAM 
SHOWING  THE  RE- 
LATION OF  PARTS  IN 
THE  INTESTINE  FOR 

THE   ABSORPTION    OF 
FOOD 

bv,  the  blood  vessels  in  the 
intestinal  wall; 

in,  the  intestinal  cavity 
occupied  by  the  digested 
food; 

m,  the  membrane  of  the 
epithelial  cell  through  which 
the  food  dialyzes  into  the 
blood  vessels. 


THE  MECHANICS  OF  THE  LIVING  MACHINE  309 

complex  than  has  been  considered.  As  the  food  is  being  passed 
through  the  intestinal  walls  it  is  changed  further  in  its  chemi- 
cal nature,  and  by  the  time  it  has  reached  the  blood  it  is  in 
a  different  chemical  state  from  that  in  which  it  left  the  intes- 
tines. 

While,  therefore,  osmosis  is  the  fundamental  factor  concerned 
in  the  absorption  of  food,  we  are  obliged  to  admit  that  it  is 
not  the  only  factor  concerned,  and  that  there  are  some  phases 
of  the  food  absorption  that  we  do  not  yet  understand.  At 
the  present  time  we  may  speak  of  this  unknown  factor  as  the 
vital  factor  of  food  absorption.  By  this  term  " vital  factor"  we 
simply  mean  the  undiscovered  forces  concerned.  No  biologist 
doubts  that  the  further  study  of  the  digestive  process  will  dis- 
close the  nature  of  these  vital  forces,  just  as  a  previous  study 
has  explained  the  early  phases  of  food  absorption.  In  other 
words,  the  general  belief  of  biologists  to-day  is  that  here  the 
term  " vital"  is  only  a  means  of  concealing  our  ignorance  of 
facts  which  are  yet  to  be  discovered.  We  have  no  reason  for 
believing  that  there  are  any  peculiar  forces  concerned  in  the 
absorption  of  food.  Modern  biology  thus  explains  the  ab- 
sorption of  food  by  the  application  of  the  same  chemical  and 
physical  forces  that  are  found  elsewhere  in  nature. 

Circulation. — The  next  function  in  animal  life  is  the  circu- 
lation of  the  blood,  which  carries  the  absorbed  food  to  the 
various  parts  of  the  body  where  it  is  needed.  The  mechanism 
of  the  circulatory  system  is  very  simple  and  is  based  upon 
mechanical  principles.  The  circulating  blood  is  contained  in 
a  series  of  tubes,  the  blood  vessels,  extending  to  every  part  of 
the  body.  At  the  center  of  this  series  of  vessels  there  is  a 
pump,  the  heart,  which  keeps  the  blood  moving.  The  heart 
is  like  a  pump,  with  valves  opening  in  one  direction  only. 
Its  structure  is  such  that  the  expansion  and  contraction  of 
its  walls  will  open  and  close  the  valves,  and  cause  the  blood 
to  flow  in  one  direction.  By  examination  of  Figure  136,  which 
represents  diagrammatically  the  structure  of  the  human  heart, 


310 


BIOLOGY 


it  will  readily  be  seen  how  the  valves  work  to  prevent  the 
backward  passage  of  the  blood,  and  to  force  it  onward  when  the 
walls  of  the  heart  contract.  The  blood  forced  from  the  heart 
is  received  in  elastic  blood  vessels,  the  arteries,  which  branch 
and  grow  smaller  as  they  pass  from  the  heart,  and  finally  break 
up  into  extremely  minute  and  even  microscopic  vessels.  After 


FIG.  136. —  DIAGRAM  OF  ONE  SIDE  OF  THE  HEART,  SHOWING 
THE  MECHANISM  OF  THE  VALVES 

A,  in  the  state  of  relaxation;  B,  at  the  time  of  contraction.  In  A  the  open  valves  admit 
the  flow  of  blood  from  the  veins  into  the  ventricles.  In  B  the  valve  connecting  with  the 
auricle  is  closed  and  the  contraction  of  the  heart  forces  the  blood  up  through  the  semiluuar 
valve,  as  is  shown  by  the  arrows.  Upon  relaxation  of  the  ventricle,  the  semilunar  valve 
closes,  and  prevents  the  flow  of  the  blood  back  into  the  ventricle,  while  the  auriculo- 
ventricular  valve  opens  and  allows  blood  to  enter  from  the  vein. 


a,  auricle; 

avv,  auriculo-ventricular  valve; 


sh,  semilunar  valve; 
v,  ventricle. 


passing  these  capillaries,  the  vessels  are  again  united  into 
larger  tubes  which,  by  combining  with  each  other,  form  the 
large  veins  that  flow  back  to  the  heart.  The  whole  action  of 
this  system  is  mechanical;  and  we  can  arrange  a  series  of 
elastic  rubber  tubes  with  a  central  beating  force-pump,  in  a 
manner  to  imitate  the  chief  functions  of  the  circulation.  Into 
the  details  of  this  matter  we  need  not  go;  for  our  purpose  it 
is  sufficient  to  understand  that  the  circulation  of  the  blood  is 
a  mechanical  phenomenon  which  can  easily  be  imitated  by 


THE  MECHANICS  OF  THE  LIVING  MACHINE 


311 


-rc 


machinery  devised  on  the  same  general  structure  as  the  heart 
and  blood  vessels. 

It  is  evident,  however,  that  one  phase  in  the  circulation 
requires  further  explanation.  The  force  that  drives  the  blood 
is  the  contraction  of  the  walls  of  the  heart.  Unless  we  ex- 
plain the  beating  of  the  heart,  we  have  not  explained  cir- 
culation. The  explanation  of  this  phenomenon  belongs  to 
the  study  of  muscles,  for  the  walls  of  the  heart  are  nothing 
more  than  a  chamber  made  up  of  a  series  of  muscles.  The 
beat  of  the  heart  is,  therefore,  no  more  mysterious  than 
the  contraction  of  other  muscles,  The  contraction  of  the 
muscles,  it  is  true,  we  do 
not  yet  fully  understand, 
but  we  do  know  that  mus- 
cles constitute  a  machine 
which  by  physical  laws 
transforms  the  energy 
stored  in  the  foods  into 
motion. 

Not  only  is  the  distribu- 
tion of  the  blood  to  be 
explained  by  mechanical 
principles,  but  the  method 
by  which  the  blood  sup- 
plies the  tissues  with  their 
nourishment  is  fairly  simple. 
The  blood  first  absorbs  nour- 
ishment from  the  alimen- 
tary canal  and  is  then  car- 
ried into  the  active  tissues  wc'  w^ite  corpuscles; 

rc,  red  corpuscles. 

of  the  body  —  for  example, 

to  the  muscles — where  again  it  is  placed  in  a  position  in  which 
osmotic  pressure  will  be  exerted.  The  blood  passes  through  the 
muscles  in  thin-walled  capillaries,  on  the  outside  of  which  is  a 
liquid  called  the  lymph  (Fig.  137),  and  thus  there  is  a  membrane 


FIG.  137. —  DIAGRAM  OF  A  FEW  CAPIL- 
LARIES FILLED  WITH  BLOOD  CORPUS- 
CLES AND  SURROUNDED  BY  LYMPH. 
THE  ARROWHEADS  SHOW  THE  DIALY- 
SES  FROM  THE  LYMPH  INTO  THE 
TISSUES,  AND  FROM  THE  TISSUES 
BACK  INTO  THE  BLOOD 


312 


BIOLOGY 


separating  two  liquids,  i.  e.,  the  capillary  walls  separating  the 
blood  and  the  lymph.  Under  these  conditions  osmosis  will 
take  place,  and  thus  the  same  general  force  which  was  con- 
cerned in  the  passage  of  the  materials  from  the  intestine  into 
the  blood,  will  cause  the  passage  of  the  same  materials  from 
the  blood  vessels  into  the  lymph  in  the  tissues.  This  lymph 
lies  in  direct  contact  with  the  living  cells,  and  these  living  cells 
can  now  take  from  the  lymph  the  food  material  that  they 
need.  This  latter  function,  by  which  the  living  cells  take  the 
material  that  they  need,  is  not  explained  by  any  known  force, 
so  we  speak  of  it  as  due  to  what  we  still  call  vital  force. 

Respiration. — The  absorption  of  oxygen  by  the  blood  in  the 
lungs  of  a  frog  or  the  gills  (branchiae)  of  a  fish,  and  the  elimi- 
nation of  the  carbon  dioxid,  are  also  processes  which  are  ex- 
plainable by  simple  chemical  laws.  The  blood  contains  certain 
substances  which  have  a  chemical  affinity  for  oxygen,  and 
others  which  have  a  chemical  affinity  for  carbon  dioxid.  The 
red  coloring  matter,  hemoglobin,  has  a  chemical  affinity  for 

oxygen,  and  will  absorb  the  gas 
whenever  it  is  in  contact  with  it, 
provided  the  pressure  of  the  oxy- 
gen is  sufficient.  But  this  union 
is  a  peculiar  one.  If  the  atmos- 
phere contains  oxygen  under 
high  pressure,  the  haemoglobin 
will  unite  with  the  oxygen,  but  if 
the  oxygen  pressure  is  low  the 
haemoglobin  will  let  go  of  the  oxy- 
gen. As  a  result,  whenever  blood 
passes  through  the  lungs,  where 
there  is  a  large  quantity  of  air  and 
where  oxygen  is  under  high  pres- 
sure, the  haemoglobin  combines 

with  oxygen;  Fig.  138.  The  blood  is  then  carried  around  the 
body,  and  when  it  reaches  the  active  tissues,  like  the  muscles, 


vein 


^artery 

FIG.  138. —  AN  AIR  SAC  OF 
THE  LUNGS 

Showing  the  blood  vessels  distributed 
in  the  wall  in  position  to  absorb  oxygen 
from  the  cavity  of  the  sac  and  excrete 
carbon  dioxid  into  it. 


THE  MECHANICS  OF  THE  LIVING  MACHINE  313 

the  glands,  or  the  brain,  it  finds  a  condition  where  there  is  prac- 
tically no  free  oxygen.  Here,  since  the  oxygen  pressure  becomes 
reduced,  the  haemoglobin  at  once  lets  go  its  hold  upon  the 
oxygen  which  it  has  seized  in  the  lungs.  The  oxygen  then 
passes  off  rapidly  into  the  tissues  and  the  blood  is  carried  back 
again  to  the  lungs  to  get  a  fresh  supply.  There  is  a  similar 
relation  between  carbon  dioxid  and  the  blood;  when  the  pressure 
of  carbon  dioxid  is  high  the  blood  will  absorb  it,  and  when  the 
pressure  is  low,  the  blood  will  let  go  its  hold  upon  the  carbon 
dioxid  it  has  absorbed.  In  the  active  tissues  and  cells,  carbon 
dioxid  is  present  in  considerable  quantity,  as  the  result  of  the 
activity  of  the  tissues.  When  the  blood  flows  through  these 
tissues,  it  therefore  absorbs  carbon  dioxid,  and  then  goes  back 
to  the  lungs  loaded  with  this  gas.  In  the  lungs,  however,  it 
comes  in  contact  with  the  air,  in  which  the  carbon  dioxid  is 
present  in  very  small  quantities  only.  Under  these  circum- 
stances the  blood  can  no  longer  hold  the  carbon  dioxid.  This 
gas  passes  into  the  lungs  and  is  exhaled  in  the  next  breath. 
These  two  processes  are  purely  chemical;  they  will  take  place 
just  as  well  in  a  laboratory  as  in  the  lungs,  and  are  quite 
independent  of  any  vital  factors. 

Up  to  this  point  in  the  study  of  the  activity  of  the  living 
body,  there  is  no  special  difficulty  in  reaching  the  following 
conclusions:  (1)  So  far  as  relates  to  the  general  problem  of 
the  transformation  of  energy,  the  body  neither  creates  nor 
destroys  energy,  but  simply  transforms  one  kind  into  another. 
(2)  So  far  as  concerns  the  functions  now  considered,  the 
laws  of  chemistry  and  physics  furnish  for  them  an  adequate 
explanation. 

It  is  necessary,  however,  to  question  further  a  function  of 
life  in  which  the  mechanical  relation  is  less  obvious.  The 
nervous  system  controls  all  the  operations  of  the  body  as  an 
engineer  controls  an  engine.  Is  it  possible  that  this  phase  of 
living  activity  can  be  included  within  the  conception  of  the 
body  as  a  living  machine? 


314  BIOLOGY 

The  Nervous  Functions. —  The  primary  question  is  whether 
there  is  any  correlation  between  nervous  force  and  other  types 
of  energy.  For  this  purpose  it  will  be  convenient  to  separate 
the  phenomena  of  simple  nervous  transmission  from  those 
that  we  speak  of  as  mental  phenomena.  The  former  are  sim- 
pler and  offer  the  greater  hope  of  solution. 

Nerve  impulse. — If  we  are  to  find  any  correlation  between 
nervous  force  and  physical  energy,  it  must  be  done  by  find- 
ing some  way  of  measuring  nervous  energy  and  comparing 
it  with  physical  energy.  There  has  been  devised  as  yet  no  satis- 
factory way  of  measuring  the  nervous  impulse  directly.  In  the 
experiment  of  keeping  an  individual  in  a  large  box  where  all  of 
the  energy  exhibited  by  his  body  can  be  carefully  and  accurately 
measured,  the  attempt  has  been  made  to  get  some  indication  of 
the  energy  involved  in  nervous  phenomena.  But  the  results 
have  been  quite  negative.  When  in  these  boxes  an  individual 
simply  arises  from  his  chair,  the  measuring  device  of  the  ap- 
paratus is  accurate  enough  to  show  distinct  indication  of  the 
expenditure  of  energy  in  this  very  simple  motion.  But  when 
this  person  is  allowed  to  remain  seated,  not  performing  any 
bodily  action,  but  working  hard  with  his  brain,  as  for  example 
in  writing  a  difficult  examination,  there  seems  to  be  exhibited 
no  extra  energy,  so  far  as  can  be  determined  by  the  measure- 
ment recorded  with  this  apparatus.  In  spite  of  all  attempts 
that  have  been  made,  it  has  hitherto  been  impossible  to  get  any 
indication  that  the  use  of  the  nervous  system  involves  the  ex- 
penditure of  energy.  This  is  probably  due  to  the  fact  that 
the  amount  of  energy  thus  involved  is  altogether  too  small  to  be 
recorded  in  the  coarse  apparatus  which  has  been  devised  for  use 
in  these  experiments. 

That  there  is  some  correlation  between  nervous  force  and 
physical  energy  is  fairly  well  proved  by  experiments  along 
various  lines.  The  impulse  that  passes  along  nerves  may  be 
excited  by  a  variety  of  forms  of  ordinary  energy.  Any  mechani- 
cal shock,  a  little  heat,  or  an  electrical  shock  will  develop  a 


THE  MECHANICS  OF  THE  LIVING  MACHINE  315 

nervous  impulse.  Now,  if  forms  of  physical  energy  applied  to 
a  nerve  are  capable  of  giving  rise  to  a  nerve  stimulus,  the 
inference  is  certainly  a  legitimate  one  that  the  nerve  is  simply 
a  bit  of  machinery  which  converts  one  kind  of  energy  into 
another,  i.  e.,  converts  physical  energy  into  nervous  energy. 
If  this  be  the  case,  of  course  it  is  necessary  for  us  to  regard 
nervous  force  as  one  of  the  correlated  forms  of  energy. 

Other  facts  point  in  the  same  direction.  Not  only  can  the 
nerve  stimulus  be  developed  by  an  electric  shock,  but  the 
strength  of  the  stimulus  is,  within  certain  limits,  proportional 
to  the  strength  of  the  shock  producing  it.  Conversely,  we  also 
find  that  a  nerve  stimulus  produces  electrical  energy.  In  an 
ordinary  nerve,  even  when  it  is  not  active,  there  are  slight 
electric  currents  that  can  be  detected  by  very  delicate  appa- 
ratus. If  the  nerve  is  stimulated,  these  electric  currents  are 
immediately  affected  in  such  a  way  that  they  may  be  increased 
or  decreased  in  intensity.  These  variations  in  intensity  are 
sufficient  to  be  visible  by  delicate  apparatus,  and  by  using  a 
galvanometer  we  can  actually  measure  the  passage  of  an  im- 
pulse passing  along  a  nerve  like  a  wave,  and  can  approximately 
determine  the  shape  of  the  wave. 

Since  the  nervous  impulse  can  be  started  by  some  other 
form  of  energy,  and  since  in  turn  it  can  modify  ordinary  forms 
of  energy,  we  cannot  avoid  the  conclusion  that  the  nervous 
impulse  is  a  special  form  of  energy  developed  within  the  nerves. 
It  is  possibly  a  form  of  wave  motion,  peculiar  to  the  nerve 
substance,  but  correlated  with  and  developed  by  other  types 
of  energy.  This  of  course  would  make  the  nerve  fiber  a  simple 
bit  of  machinery. 

If  this  conclusion  is  correct,  it  will  follow  that  whenever  a 
nerve  impulse  passes  over  a  nerve  a  certain  portion  of  the  food 
supply  in  the  nerve  must  be  broken  to  pieces  to  liberate  energy, 
and  this  would  also  be  accompanied  by  the  elimination  of 
carbon  dioxid  and  heat.  But  although  careful  experiments 
have  been  made,  it  is  as  yet  impossible  to  detect  any  rise  in 


316  BIOLOGY 

temperature  when  a  nerve  impulse  passes  over  a  nerve.  This 
is  not,  however,  an  objection  to  the  general  theory,  since  the 
nerve  is  such  a  small  machine  that  it  would  be  doubtful  whether 
our  tests  are  delicate  enough  to  recognize  any  rise  in  tempera- 
ture even  if  such  a  rise  occurred.  The  total  energy  of  the 
nervous  impulse  is  too  small  to  be  detected  by  our  rough 
instruments  for  measuring  heat. 

All  evidence  goes  to  show  that  the  nervous  impulse  is  a 
form  of  motion,  and  hence  is  correlated  with  other  forms  of 
physical  energy.  The  nerve  is  a  very  delicate  machine  and  its 
total  amount  of  energy  is  very  small.  A  tiny  watch  is  more 
delicate  than  a  water-wheel,  and  its  actions  are  more  closely 
dependent  upon  the  accuracy  of  its  adjustment.  The  water- 
wheel  may  be  made  very  coarsely  and  still  be  useful,  while  the 
watch  must  be  fashioned  with  extreme  care  and  nicety.  Yet 
the  water-wheel  transforms  vastly  more  energy  than  the  watch ; 
it  may  drive  the  machinery  of  the  whole  factory,  while  the 
watch  can  no  more  than  move  itself.  But  who  can  doubt  that 
the  watch  as  well  as  the  water-wheel  is  governed  by  the  law 
of  the  correlation  of  forces?  So  the  nerve  machine  of  the  living 
body  is  delicately  adjusted,  easily  put  out  of  order,  and  its 
actions  involve  only  a  small  amount  of  energy;  but  it  is  prob- 
ably just  as  truly  subject  to  the  law  of  the  conservation  of 
energy  as  are  the  more  massive  muscles. 

Sensations. — Up  to  a  certain  point,  sensations  can  also  be 
related  to  the  general  problem  of  the  conservation  of  energy. 
The  frog  has  a  piece  of  apparatus,  which  we  call  the  ear,  capable 
of  being  affected  by  the  vibrating  waves  of  the  air.  It  is  made 
of  parts  so  delicately  adjusted  that  the  air  waves  set  them  in 
motion,  and  this  motion  starts  a  nervous  stimulus  which  travels 
along  the  auditory  nerve  to  the  brain.  Whenever  air  waves 
strike  the  frog's  ear,  they  will  excite  in  his  auditory  nerve 
impulses  which  will  travel  from  the  ear  to  the  brain.  The  ear 
is  simply  a  delicately  poised  apparatus,  so  adjusted  that  when 
it  is  stimulated  by  vibrating  air  it  is  discharged  like  a  bit  of 


THE  MECHANICS  OF  THE  LIVING  MACHINE  317 

gunpowder,  and  a  nervous  impulse  is  produced.  In  all  of  this 
we  are  plainly  dealing  with  nothing  more  than  the  transforma- 
tion of  one  type  of  energy  into  another.  In  the  same  way  the 
optic  nerve  has  at  its  end,  in  the  eye,  a  bit  of  mechanism  that 
is  easily  excited  by  the  light  waves,  and  when  such  waves  strike 
the  eye  there  will  be  started  in  the  optic  nerve  a  series  of 
impulses  which  pass  towards  the  brain.  Thus  each  sensory 
nerve  has  at  its  end  a  bit  of  machinery  designed  for  trans- 
forming certain  kinds  of  external  force  into  nervous  impulses. 

The  second  phase  of  the  sensation  is,  on  the  other  hand, 
not  explainable  by  any  mechanical  principle.  When  the  im- 
pulse started  in  the  ear  reaches  the  brain,  it  is  converted  into 
what  we  call  a  sensation,  i.  e.,  a  consciousness,  a  perception,  a 
distinct  feeling.  In  our  attempt  to  trace  external  forces  we  can 
follow  the  stimulus  to  the  brain,  but  there  we  must  stop.  We 
have  no  idea  how  a  nervous  impulse  is  converted  into  sensation. 
By  no  means  of  thinking  can  we  conceive  of  the  correlation 
of  the  sensation  itself  with  any  form  of  physical  energy.  It 
is  true  that  the  mental  sensation  is  excited  by  the  nervous 
impulse,  and  true  also  that  in  the  development  of  the  individual 
the  mental  powers  develop  parallel  with  the  growth  of  the 
nerves  and  brain.  Moreover,  certain  visible  changes  occur  in 
the  brain  cells  when  they  are  excited  into  mental  activity. 
All  of  these  facts  point  to  a  close  association  between  the  mental 
side  of  sensation  and  the  physical  structure  of  the  machine. 
But  they  do  not  prove  any  correlation  between  them.  The 
unlikeness  between  the  mental  and  physical  phenomena  is  so 
absolute  that  we  must  hesitate  about  drawing  any  connection 
between  them.  It  is  impossible  to  conceive  of  the  mental  side 
of  sensation  as  a  form  of  wave  motion. 

Mental  functions. — If  we  go  farther  and  try  to  consider  the 
other  phenomena  associated  with  the  nervous  system — the  more 
distinctive  mental  processes — we  have  absolutely  no  ground  for 
comparison.  We  cannot  imagine  thought  measured  by  units; 
and  until  we  conceive  of  some  such  measurement  we  can  get 


318  BIOLOGY 

no  meaning  from  any  attempt  to  find  correlation  between  the 
true  mental  processes  and  physical  energy. 

Reproduction. —  The  process  of  reproduction  would  seem  to 
be  one  which  cannot  possibly  be  explained  as  the  result  of 
chemical  and  physical  forces.  Nowhere  else  in  nature  do  we 
find  this  property,  and  in  this  respect  living  organisms  cannot 
be  compared  to  any  other  machine.  Nevertheless,  in  its  sim- 
plest form  reproduction  also  permits  a  partial  explanation. 
When  a  unicellular  organism,  like  the  Amoeba  (Fig.  19),  feeds 
and  grows,  it  increases  in  size.  The  increase  in  size  is  due  to 
the  transformation  of  the  chemical  material  of  its  food  into  a 
material  like  that  of  the  animal,  and  as  these  new  materials 
accumulate,  the  bulk  of  the  animal  becomes  greater.  As  the 
animal  increases  in  bulk,  it  needs  a  larger  supply  of  oxygen  to 
keep  up  its  life  processes,  since  all  life  processes  require  the 
expenditure  of  oxygen,  and  the  amount  of  oxygen  needed  is 
dependent  on  the  bulk  of  the  animal  that  is  to  be  supplied. 
Now  it  is  a  principle  of  mathematics  that  the  bulk  of  a  solid 
object  increases  as  the  cube  of  its  dimensions,  whereas  its  sur- 
face increases  only  as  its  square.  Since  this  Amoeba  is  obliged 
to  absorb  all  of  its  oxygen  through  the  surface  of  its  body,  it 
follows  that  the  surface  adapted  for  absorbing  of  oxygen  in- 
creases only  as  the  square  of  its  diameter,  while  its  need  for 
oxygen  increases  as  the  cube.  It  is  evident  from  this  that  in 
time  the  surface  will  no  longer  be  sufficient  to  absorb  enough 
oxygen  for  its  increasing  size.  When  this  time  comes  the  ani- 
mal must  either  stop  growing  or  devise  some  way  of  increasing 
its  absorptive  surface.  What  happens  is  that  the  bit  of  living 
jelly  simply  breaks  in  two.  The  result  is  that  once  more  the 
absorbing  surface  is  large  enough  to  accommodate  a  larger 
bulk,  and  the  animal  again  begins  to  grow.  This  explanation 
of  reproduction  shows  how  the  process  may  have  been  due 
to  overgrowth.  Since  all  kinds  of  reproduction  are  forms  of 
division,  it  follows  that  if  we  can  explain  the  simplest  division 
upon  the  basis  of  physical  and  chemical  forces,  we  have  at 


THE  MECHANICS  OF  THE  LIVING  MACHINE  319 

least  reached  an  intelligible  understanding  of  the  process. 
The  more  complicated  phases  of  reproduction  are,  of  course, 
not  explained  by  this  simple  process,  not  even  the  division  of 
a  cell  which  we  have  seen  to  be  very  complicated;  but  if  we 
can  explain  this  strange  phenomenon  even  in  its  simplest  form, 
we  have  done  much  toward  explaining  the  functions  of  repro- 
duction in  accordance  with  the  principle  of  chemical  and  phys- 
ical forces. 

VITAL  FORCE  OR  VITALITY 

With  all  of  the  explanation  given,  we  cannot  believe  that 
we  have  reached  a  solution  of  life.  There  is  clearly  something 
lacking,  for  we  still  have  to  ask  the  question  why  it  is  that  all 
of  these  chemical  and  physical  forces  play  together  in  such 
harmony  within  the  living  organism.  Nowhere  in  nature  can 
the  physical  forces  automatically  carry  on  such  functions  except 
in  living  organisms.  It  is  quite  possible  to  compare  the  animal 
body  to  a  locomotive  at  rest.  But  a  locomotive  at  rest,  even 
if  its  boilers  are  filled  with  steam  under  high  pressure,  ,will 
never  exhibit  any  activity  without  an  engineer  to  control  the 
forces  that  are  contained  in  the  machine.  The  living  organism 
has  no  outside  engineer.  What  is  there  in  the  living  organism 
that  corresponds  to  the  engineer  starting  and  directing  the 
machinery?  To  this  question  we  have  no  answer.  Some  bi- 
ologists claim  that  there  is  no  more  need  of  an  engineer  for  a 
living  organism  than  for  a  clock,  these  scientists  assuming 
that  the  complexity  of  the  machine  gives  it  automatic  activity. 
Others  would  believe  that  in  a  living  being  there  is  something 
that  is  absent  in  other  machines,  to  which  they  would  give  the 
distinct  name  of  vitality.  There  are  certain  functions  of  this 
machine,  like  sensation,  thought,  etc.,  that  do  not  seem  to  be 
explainable  by  chemical  and  physical  laws,  and  one  class  of 
biologists  would  group  these  functions  together  under  the 
general  term  of  vitality.  Others  would  claim  that  vitality 
has  no  real  meaning,  but  is  only  a  name  given  to  a  combination 
of  functions  possessed  by  certain  machines.  The  question 


320  BIOLOGY 

whether  there  is  anything  like  vital  force  has  not  yet  been 
solved,  and  it  is  by  no  means  certain  that  it  ever  will  be. 
If  it  were  possible  for  scientists  to  manufacture  a  cell  exhibit- 
ing the  properties  of  life,  the  great  problem  of  biology  would 
be  settled.  This  has  never  been  done,  and  we  must  leave  the 
question  of  the  meaning  of  vitality  without  an  answer.  It 
cannot  be  insisted  upon  too  strongly  that,  while  we  may 
compare  the  living  organism  with  a  machine,  it  is  unlike  any 
other  machine.  The  living  machine  consists  of  a  number  of 
small  independent  units  called  cells,  each  one  of  which  has 
its  own  independent  power  of  growth  and  reproduction.  The 
whole  combination,  too,  has  functions  possessed  by  no  other 
machine. 

Complex  and  Simple  Living  Machines. — An  animal  as  high 
in  the  scale  as  the  frog  is  evidently  an  extremely  complicated 
machine.  Not  only  is  it  made  up  of  a  large  number  of  parts, 
each  with  a  different  function,  but  each  of  these  parts  is  made 
up  of  a  number  of  tissues,  each  having  a  different  relation  to 
the  organ  in  general;  and  furthermore,  each  of  these  tissues 
is  made  up  of  hundreds,  thousands,  and  perhaps  millions  of 
living  units,  called  cells.  It  seems  plausible  to  think  that,  if 
we  could  get  rid  of  the  complexity  seen  in  the  frog,  we  might 
approach  nearer  to  primitive  life.  In  other  words,  if  we  can 
get  at  the  simplest  unit  of  life  we  might  be  able  to  understand 
many  mysterious  phenomena,  since  we  should  thus  approach 
life  in  its  simplest  form.  For  this  purpose  biologists  have  turned 
especial  attention  to  the  life  of  the  individual  cell,  since  this  is 
the  simplest  known  unit  manifesting  life.  It  is  clear,  however, 
from  the  study  of  cells  in  Chapter  II,  that  the  mysteries  of 
life  phenomena  are  not  solved  by  reducing  them  to  the  opera- 
tions going  on  inside  of  the  single  cell.  Although  some  cells 
are  simpler  than  the  one  shown  in  Figure  9,  still  it  represents 
practically  the  simplest  form  of  machinery  with  which  we  are 
acquainted  that  is  capable  of  carrying  on  the  functions  of  life. 
But  such  a  cell  itself  is  a  complex  machine,  and  if  we  study  in 


THE  MECHANICS  OF  THE  LIVING  MACHINE          321 

it  the  processes  of  life,  it  becomes  evident  that  the  functions 
of  this  single  machine  are  as  mysterious,  although  not  so  com- 
plex, as  are  the  functions  of  the  whole  body  of  the  frog.  In 
other  words,  getting  rid  of  the  complex  machinery  of  such  a 
highly  built  organism  as  the  frog  does  not  help  us  at  all  towards 
the  [solution  of  the'  problems  of  biology;  for  it  is  no  easier 
to  understand  the  processes  of  life  going  on  in  the  single  cell 
than  it  is  to  understand  the  processes  of  life  going  on  in  the 
multicellular  animal.  While  the  study  of  single  cells  and  their 
functions  has  enabled  us  to  understand  the  processes  'of  life 
in  many  respects  much  better  than  before,  it  has  not  solved 
the  problem  of  what  life  is,  nor  made  it  any  easier  to  get 
rid  of  the  idea  that  living  organisms  show  certain  powers 
not  possessed  by  machines,  —  powers  so  mysterious  that  we 
must  acknowledge  our  inability  to  explain  them,  and  must, 
for  the  present  at  least,  include  them  under  the  general  term 
of  vitality. 

The  recognition  that  the  cell  is  such  a  complex  mechanism 
has  recently  led  to  the  attempt  to  analyze  it  into  smaller  and 
simpler  units.  Whether  any  success  will  follow  this  attempt 
it  is  too  early  to  predict. 

For  these  reasons  it  is  useful  still  to  retain  the  term  "vital 
force";  not  meaning  by  this  to  imply  that  there  is  any  special 
force  in  living  things,  uncorrelated  to  forces  of  nature,  but 
simply  indicating  our  present  lack  of  knowledge.  By  vitality 
we  refer  to  the  guiding  principles  which  regulate  the  play  of 
chemical  and  physical  forces  in  this  living  machine,  and  which 
determine  the  processes  of  reproduction,  which  lie  at  the  foun- 
dation of  that  side  of  living  organisms  and  their  functions  which 
we  call  mental.  We  certainly  have  not  yet  explained  all  the 
factors  connected  with  life  processes,  and  we  can  therefore 
most  satisfactorily  comprehend  them  under  the  term  "vitality." 
With  this  understanding,  it  is  perfectly  legitimate  to  retain  th* 
term  "vital  force"  for  those  phases  of  life  processes  which  are 
not  included  in  any  mechanical  conception  of  life. 


322  BIOLOGY 

SUMMARY 

1.  All  physical  energy  exerted  by  the  living  organism  is 
distinctly  correlated  with  other  forms  of  energy,  the  energy  of 
plants  coming  from  sunlight,  and  that  of  animals  coming  from 
the  energy  stored  by  plants  in  their  foods.  To  this  extent, 
therefore,  a  living  organism  is  a  machine.  2.  Nearly  all  life 
functions  are  explainable  by  chemical  and  physical  laws.  This  is 
certainly  true  of  such  functions  as  digestion,  assimilation,  circu- 
lation, excretion,  respiration,  etc.  3.  Some  of  the  functions  of 
the  living  animal  are  not  yet  explained  by  chemical  or  physical 
forces.  This  is  true  of  the  absorption  of  the  food  from  the 
intestines,  and  the  power  which  the  living  cells  have  of  taking 
from  the  lymph  the  particular  form  of  food  that  they  need. 
We  may  gather  these  factors  for  the  present  under  the  term 
"vital  forces"  of  the  living  organism.  After  we  have  learned 
thoroughly  to  understand  them  and  their  method  of  action, 
we  may  find  these  processes  are  also  to  be  included  under  the 
general  laws  of  physics  and  chemistry.  There  is  really  no  good 
reason  for  questioning  that  the  living  organism  is  a  mechanism, 
simply  because  there  are  some  functions  which  are  at  present 
unintelligible.  4.  In  the  mental  power  of  the  living  organism 
appear  functions  which  are  not  found  in  any  machine.  The 
functions  of  mind,  sensation,  and  thought  are  so  absolutely 
unique,  and  so  different  from  any  other  type  of  energy,  that 
no  one  has  ever  conceived  the  possibility  of  correlating  them 
with  physical  energy.  5.  Only  the  living  machine  has  the 
power  of  reproducing  itself.  It  is  true  that  some  forms  of  the 
process  of  reproduction  may  be  explained  simply  as  a  result 
of  growth,  and  growth  as  due  to  the  chemical  forces  that  are 
at  play  within  the  living  organism.  But  it  nevertheless  remains 
true  that  no  other  mechanism  in  nature  has  the  power  of  divid- 
ing itself  into  two  parts,  each  of  which  develops  into  an  indi- 
vidual like  the  first.  Taking  all  these  things  into  considera- 
tion, it  is  evident  that,  so  far  as  physical  forces  are  concerned, 
the  living  organism  is  a  machine,  and,  like  other  mechanisms, 


THE  MECHANICS  OF  THE  LIVING  MACHINE          323 

transforms  one  type  of  energy  into  another.  But  the  living 
organism  possesses  additional  powers,  some  of  which  may  be 
explained  some  day,  while  others,  like  thought  and  reproduc- 
tion, appear  to  be  insoluble  and  place  the  living  organism 
in  a  category  by  itself.  If  the  living  organism  is  a  machine, 
it  is  also  more  than  a  machine,  and  cannot  be  compared  with 
any  other  mechanism  in  nature. 

WHAT  IS  LIFE? 

It  may  be  instructive  to  ask  whether  we  can  define  life. 
Although  many  attempts  have  been  made  to  give  the  defini- 
tion of  life,  all  that  can  be  done  is  to  describe  some  of  its  char- 
acteristics. The  primary  characteristic  of  living  things  is  a 
constant  activity,  and  if  we  mean  anything  by  the  term  "life," 
it  must  be  the  guiding  force  that  controls  these  activities. 
Our  understanding  of  the  word  "life"  is  certainly  obscure; 
but,  so  far  as  it  means  anything,  it  refers  to  the  engineer  that 
controls  the  engine,  the  machinist  that  directs  the  activity  of 
the  machine,  the  force  that  guides  the  activities  of  the  animals 
or  plants.  What  this  guiding  force  is  we  do  not  know.  Some 
have  called  it  "vital  force,"  and  have  believed  it  to  be  a  special 
force  in  nature.  Others  insist  that  there  is  no  special  force  in 
living  things,  any  more  than  there  is  in  a  clock  or  a  watch. 
Whether  there  is  any  force  in  nature  that  can  properly  be  called 
vitality  is  not  yet  settled,  but  it  is  certain  that  the  phenomenon 
which  we  call  life  is  manifested  only  in  those  machines  which 
we  call  animals  and  plants,  and  which  come  from  no  source 
except  that  of  previously  existing  animals  and  plants.  We  have 
no  evidence  that  this  force  can  be  created  in  any  way  except 
from  life  which  previously  existed.  The  life  force  is  capable 
of  indefinite  growth  and  expansion,  since  a  fraction  of  life 
force,  in  the  form  of  any  single  animal,  may  produce  hundreds 
of  thousands  of  offspring,  each  of  which  has  the  same  amount 
of  life  force  as  the  original  ancestor  had.  But  this  life  force, 
although  capable  of  expansion  and  growth,  has,  so  far  as  we 


324  BIOLOGY 

know,  no  method  of  origin  except  from  previously  existing  life. 
We  must  look  at  life  as  a  unique  manifestation  of  force,  stand- 
ing by  itself.  This  is  perfectly  consistent  with  the  recognition 
of  the  fact  that  the  animal  body  is  a  machine,  acting  in  accord- 
ance with  the  principles  of  conservation  of  energy,  and  that 
a  living  organism  simply  transforms  one  type  of  energy  into 
another.  This  view  is  also  equally  consistent  with  the  sug- 
gestion that  there  is  a  special  force,  which  we  call  life,  directing 
the  activity  of  these  machines.  At  all  events,  for  the  present 
we  can  go  no  farther  in  the  discussion  of  the  question  than  this. 
Life  is  the  directive  agent  which  controls  the  activity  of  the 
living  machine,  and  death  means  the  disappearance  of  this 
controlling  agent;  though  what  is  meant  by  its  disappearance 
we  cannot  say,  any  more  than  we  can  tell  what  caused  its 
appearance  in  the  machine  in  the  first  place.  The  question  of 
the  real  significance  of  life  and  death  is  still  unanswered  by 
science. 


CHAPTER  XVII 

THE   ORIGIN   AND   DEVELOPMENT   OF   ORGANISMS: 
HEREDITY  AND  VARIATION 

THE  ORIGIN  OF  THE  LIVING  MACHINE  NOT  EXPLAINED 

EVEN  if  it  were  possible  to  explain  perfectly  the  working  of 
the  organic  machine  by  mechanical  principles,  this  would  not 
explain  life.  As  we  have  noticed  in  Chapter  I,  living  organisms 
come  into  existence  to-day  only  as  the  result  of  reproduction 
from  previously  existing  organisms.  Granting  that  animals 
and  plants  have  the  power  of  reproduction,  we  have  still  to 
ask  how  these  complicated  machines  came  into  existence. 
One  of  the  most  revolutionary  eras  of  thought  has  arisen  in 
the  last  fifty  years  as  the  result  of  the  attempt  of  biologists 
to  explain  how  the  innumerable  animals  and  plants  have  been 
brought  to  their  present  condition  of  existence. 

Of  the  primal  origin  of  life  we  have  no  knowledge,  and  it 
must  be  admitted  we  have  little  hope  of  ever  gaining  any. 
Nor  have  we  much  idea  of  the  first  living  things  that  appeared 
in  the  world.  Probably  they  were  of  the  lowest  type,  possibly 
even  simpler  than  unicellular  forms.  One  thing  seems  certain : 
the  first  living  things  must  have  been  endowed  with  the  prop- 
erties of  growth  and  reproduction;  for  without  these  powers 
they  would  not  have  been  alive.  We  know  of  nothing  simpler 
than  cells  possessing  these  powers,  and  we  cannot  therefore  con- 
ceive the  beginning  of  life  as  anything  simpler  than  a  bit  of 
reproducing  protoplasm. 

THE  FORCES  WHICH  HAVE  PRODUCED  ORGANISMS 

It  has  been  the  aim  of  biology  to  show  how  the  endless 
series  of  complicated  animals  and  plants,  now  found  in  the 
world,  have  been  produced  from  the  simplest  forms  of  life. 

325 


326  BIOLOGY 

Living  organisms  possess  three  properties,  by  the  interaction 
of  which  the  present  world  has  been  formed.  These  are  re- 
production, heredity,  and  variability.  That  these  three  factors 
are  necessarily  concerned  is  evident.  Without  reproduction 
there  could  not  have  been  produced  the  successive  generations 
which  have  followed  each  other;  unless  the  successive  genera- 
tions had,  by  heredity,  reproduced  the  characters  of  preceding 
generations,  there  would  have  been  no  connection  between  one 
type  and  another;  and  lastly,  if  the  successive  generations  had 
not  shown  variability,  organisms  would  have  remained  in  a  sta- 
tionary condition,  without  any  opportunity  for  change.  That 
these  forces  have  been  sufficient  to  account  for  the  develop- 
ment of  the  organisms  inhabiting  the  world,  i.  e.,  to  explain 
the  origin  of  the  living  machines,  is  not  so  evident.  To  show 
how  the  result  has  been  brought  about  has  been  the  endeavor 
of  biological  discussion  for  the  last  half-century.  The  property 
of  reproduction  we  have  already  considered.  The  considera- 
tion of  heredity  and  variation  remains. 

Heredity. — The  general  rule  in  reproduction  is  that  the  off- 
spring grow  into  individuals  like  their  parents,  the  repetition 
of  the  parent  being  spoken  of  under  the  name  of  heredity. 
Heredity  must  not  be  looked  upon  as  any  special  force  or  law, 
but  merely  as  a  word  expressive  of  the  fact  that  one  generation 
repeats  itself  in  the  next.  It  is  evident  that  this  process  of 
repetition  cannot  be  exact,  since  most  animals  have  not  one 
but  two  parents,  and  an  individual  that  has  a  father  and  a 
mother  cannot  be  exactly  like  both  of  them  if  they  are  in  the 
slightest  degree  unlike.  Since  no  two  animals  are  exactly  alike, 
the  natural  conclusion  would  be  that  the  offspring  would  be  a 
compromise  between  its  two  parents.  Successive  generations 
are  thus  not  identical,  but  constantly  show  differences  from 
their  parents.  Heredity  means,  then,  that  successive  genera- 
tions resemble  their  parents  as  closely  as  is  compatible  with 
the  fact  that  the  individual  has  two  parents,  and  cannot  be 
like  both. 


THE  ORIGIN  AND  DEVELOPMENT  OF  ORGANISMS     327 

Variation. — The  offspring  of  any  animal  is  never  exactly  like 
either  of  its  parents.  Sometimes  it  is  a  compromise  between 
them;  sometimes,  for  certain  reasons  that  we  do  not  under- 
stand, it  is  quite  different  from  either.  The  reasons  why  any 
peculiarity  may  reappear  in  successive  generations,  are  probably 
partly  due  to  processes  connected  with  the  reproductive  func- 
tions, but  they  are  also  partly  due  to  the  effect  of  the  environ- 
ment in  which  the  animal  lives,  upon  the  structure,  the  nature, 
and  the  life  of  the  organism.  Whatever  be  their  cause,  the 
points  in  which  animals  and  plants  differ  from  each  other,  or 
from  their  ancestral  types,  are  known  under  the  general  name 
of  variations. 

The  life  of  an  individual  which  is  produced  by  sexual  repro- 
duction may  be  said  to  begin  at  the  moment  when  the  sperm 
fuses  with  the  egg,  as  shown  in  Figure  121.  Previous  to  this, 
there  were  only  the  sex  cells  produced  by  two  parents;  but 
from  this  point  there  is  a  new  individual  resulting  from  the 
union.  Variations  which  appear  in  an  animal  or  a  plant  may 
be  caused  by  influences  acting  either  before  or  after  the  union 
of  the  sex  cells.  If  the  variation  is  caused  by  influences  acting 
before  this  union,  we  speak  of  it  as  a  congenital  variation  (Lat. 
con  =  together  +  genitus  =  produced) .  If,  however,  the  varia- 
tion is  developed  in  the  animal  after  the  fusion  of  the  sex  cells, 
and  thus  produced  by  influences  acting  directly  on  the  new 
individual,  we  speak  of  it  as  an  acquired  variation.  Although 
this  distinction  between  acquired  and  congenital  variations  may 
be  merely  a  matter  of  a  short  time,  nevertheless  the  facts 
show  that  there  is  a  very  great  distinction  between  character- 
istics produced  before  and  after  this  period.  Variations  which 
are  produced  by  influences  acting  before  the  fusion  of  the 
sex  cells  (congenital  variations)  are  practically  certain  to  be 
handed  to  the  subsequent  generations  by  heredity.  Variations 
which  arise  subsequently,  and  affect  the  new  individual  only 
(acquired  variations),  are  practically  certain  not  to  be  handed 
on  to  the  following  generations  by  heredity. 


328  BIOLOGY 

CONFORMITY  TO  TYPE 

Nothing  is  more  marvelous,  and  at  the  same  time  more 
evident,  than  the  fact  that  the  individuals  of  generation  after 
generation  resemble  each  other  so  closely.  Not  only  in  general 
features,  such  as  the  .structure  of  the  body,  the  presence  of 
the  proper  number  of  legs,  arms,  etc.,  does  the  child  resemble 
the  parent,  but  in  an  infinite  number  of  details, —  in  the  color 
of  the  eyes,  the  color  of  the  hair,  and  even  in  many  obscure 
traits.  The  child  may  inherit  from  its  parents  the  tendency 
to  become  bald-headed  at  a  certain  age,  or  at  a  certain  time  in 
life  to  put  on  a  large  amount  of  fat,  etc.  Through  an  endless 
series  of  details,  the  child  has  a  tendency  to  repeat  its  parents' 
characteristics. 

Since  scientists  have  begun  to  study  life  phenomena,  they 
have  always  puzzled  over  these  marvelous  facts,  and  have 
advanced  many  speculations  and  theories  to  explain  the  simi- 
larity of  the  offspring  to  its  parents.  Some  of  these  theories 
have  been  ingenious,  some  have  been  plausible,  but  all  have 
been  imaginative.  For  the  last  century,  particularly,  this  sub- 
ject has  been  a  matter  of  speculation;  but  until  about  1884 
none  of  the  various  speculations  had  sufficient  plausibility  to 
receive  any  general  acceptance. 

In  1884  there  appeared  a  little  essay  by  August  Weismann 
entitled  "On  Heredity"  which  advanced  a  new  suggestion  for 
the  explanation  of  heredity.  In  some  of  its  phases  this  new 
theory  had  been  antedated  by  the  writings  of  Brooks  in  America 
and  Galton  in  England.  Nevertheless,  it  did  not  appear  in  a 
clear-cut  form  until  Weismann's  essay  came  out  in  1884,  and 
the  theory  has  been  almost  universally  known  as  Weismann's 
theory  of  heredity.  From  the  time  of  its  appearance,  the  ex- 
planation commanded  wide  acceptance  and  extended  discussion. 
As  year  by  year  has  passed,  the  theory  has  been  more  and  more 
substantiated  by  facts,  until  at  the  present  time  it  has  practi- 
cally universal  acceptance.  While  it  cannot  be  claimed  that 
we  have  a  complete  explanation  of  heredity,  it  is  beyond  ques- 


THE  ORIGIN  AND  DEVELOPMENT  OF  ORGANISMS     329 

tion  that  our  present  understanding  gives  us  an  intelligent 
grasp  of  the  law  of  conformity  to  type.  Future  experiments 
and  discussions  may  modify  our  present  ideas  in  many  details; 
but  it  is  practically  certain  that  the  fundamental  law,  in  ac- 
cordance with  which  successive  generations  tend  to  resemble 
one  another,  is  now  so  well  understood  that  it  is  not  likely  to 
be  changed  greatly  by  subsequent  investigation.  The  prin- 
ciple underlying  this  conception  of  Weismann's  is  spoken  of 
as  that  of  the  continuity  of  germ  plasm.  A  brief  resume  of 
the  theory  is  as  follows :  — 

Reproduction  by  Simple  Division. — It  is  not  difficult  to 
understand  that  when  an  animal  multiplies  by  simple  division, 
the  offspring  will  be  similar  to  each  other.  When,  for  example, 
an  Amoeba  divides,  it  would  be  almost  impossible  to  see  how 
the  two  parts  should  be  otherwise  than  alike,  since  they  are 
each  half  of  the  same  individual.  So,  too,  when  yeast  multi- 
plies by  budding,  it  is  not  difficult  to  understand  that  the  buds 
which  grow  from  the  side  of  the  older  cell  will  be  like  the  old 
cell.  If  a  cell  is  thus  capable  of  dividing,  it  would  be  very 
difficult  to  see  how  the  bud  could  in  any  degree  be  unlike  its 
parent,  except  as  it  may  be  changed  by  future  conditions. 
So,  too,  with  those  multicellular  animals  and  plants  that  mul- 
tiply by  the  process  of  budding,  the  conformity  to  type  is  nat- 
ural rather  than  marvelous.  When  Hydra  (Fig.  69)  pro- 
duces a  small  bud  on  its  side,  which  grows  to  the  size  of  the 
original  and  then  breaks  away,  it  is  not  difficult  to  see  why 
the  bud  should  be  like  the  parent,  for  it  would  be  difficult  to 
understand  how  it  could  be  otherwise.  So,  too,  when  a  branch 
of  a  tree  is  broken  off,  takes  root,  and  grows  into  a  new  tree 
when  placed  in  the  ground,  it  would  be  difficult  to  understand 
why  the  new  individual  should  be  different  from  the  parent 
tree,  since  it  is  really  a  part  of  it.  In  all  of  these  cases  the 
conformity  to  type  is  natural  and  presents  no  special  puzzle, 
beyond  the  fact  that  animals  and  plants  have  the  power 
of  dividing  and  reproducing  at  all.  Conformity  to  type  in 


330  BIOLOGY 

animals  that  multiply  by  simple  division,  or  by  budding,  ex- 
plains itself. 

Reproduction  by  Eggs  or  by  Spores. — When  we  come  to 
the  reproduction  of  the  multicellular  animals  and  plants  by 
eggs  and  spores,  the  problem,  however,  becomes  more  difficult. 
The  egg  or  the  spore  is  a  single  cell,  and  from  this  single  cell 
develops  the  many-celled  adult.  When  this  cell  divides  into 
many  cells,  which  become  differentiated  and  form  themselves 
into  new  individuals,  why  should  these  adults  be  repetitions 
of  the  parent?  There  can  be  only  one  answer.  This  single  cell, 
whether  an  egg  or  a  spore,  must  contain  in  itself,  in  some  form 
or  other,  features  representing  the  whole  of  the  animal  from 
which  it  came.  We  may  place  two  eggs  in  an  artificial  incu- 
bator and  hatch  them  by  artificial  heat  under  identical  condi- 
tions; one  of  them  becomes  a  duck  and  the  other  a  chick.  It 
is  absolutely  impossible  to  avoid  the  conclusion  that  in  one  of 
the  original  eggs  there  were  present  potentially  the  characters 
which  would  produce  the  duck,  and  in  the  other  the  charac- 
ters which  would  produce  the  chick.  This  of  course  indicates 
a  complexity  in  the  egg  far  beyond  the  possibility  of  our  imag- 
ination. But  we  are  logically  forced  to  the  conclusion  that  the 
facts  are  as  stated.  An  egg  or  a  spore  undoubtedly  contains 
potentially  all  of  the  characters  of  the  animal  into  which  it 
develops.  \ ' 

Germ  Plasm. — For  convenience  in  discussion  it  is  agreed  to 
call  this  substance,  which  is  present  in  the  egg  and  contains 
the  hereditary  characters,  by  the  name  of  germ  pksm.  We 
have  seen,  in  Chapter  XII,  reasons  for  believing  that  this  mate- 
rial is  chiefly,  if  not  wholly,  confined  to  the  part  of  the  egg  that 
we  have  called  the  chromatin.  We  have  also  learned  that  the 
chromatin  is  capable  of  growing  and  dividing  and  has  the  power 
of  self-perpetuation.  Using  the  term  "germ  plasm"  for  this 
material  that  possesses  the  power  of  determining  the  develop- 
ment of  a  new  individual,  it  follows  that  the  germ  plasm  has 
the  power  of  growth.  In  other  words,  this  germ  substance, 


THE  ORIGIN  AND  DEVELOPMENT  OF  ORGANISMS     331 

when  properly  nourished,  continues  to  increase  in  bulk  and 
may  grow  indefinitely,  becoming  more  and  more  abundant, 
but  not  essentially  changing  its  character.  If  We  admit  this 
power  of  the  germ  plasm,  the  problem  of  the  conformity  to 
type  obtains  a  ready  explanation;  for  some  of  this  germ  plasm 
is  simply  handed  on  from  one  generation  to  the  next,  constantly 
growing  in  bulk,  but  not  changing  its  character.  At  any  stage 
in  the  development  of  the  race,  there  is  present  in  each  indi- 
vidual a  certain  amount  of  germ  plasm,  containing  in  itselt 
the  general  race  characteristics.  The  way  that  this  is  brought 
about  is  believed  to  be  as  follows :  — 

In  each  egg  produced  by  an  animal  or  a  plant,  and  in  each 
sperm  produced  by  the  male,  there  is  a  small  quantity  of  this 


ov 


FIG.  139. —  DIAGRAM  TO  ILLUSTRATE  HEREDITY,  SHOWING 
TWO  GENERATIONS  OF  HYDRA 


gp,  germ  plasm; 
ov,  an  ovum; 


sp,  somaplasm; 
s,  sperm. 


The  diagram  shows  how  the  germ  plasm  in  the  egg,  ov,  divides:  one  part,  sp,  develops 
into  the  next  generation,  while  the  other  part,  the  germ  plasm,  gp,  becomes  stored  in  its 
reproductive  bodies,  ov1  and  s.  In  6,  the  germ  plasm  from  an  egg  is  combining  with  the  germ 
plasm  from  the  sperm,  s1,  in  sexual  fertilization. 

germ  plasm;  Fig.  139  gp.  It  is  the  presence  of  this  germ  plasm 
that  makes  it  possible  for  the  egg  to  develop  into  a  new  individ- 
ual like  the  parent.  An  early  step  in  the  development  of  this  egg 
toward  the  adult  consists  in  the  division  of  this  germ  substance 
into  two  parts.  The  two  are  essentially  alike  and  both  contain 
the  same  characteristics;  but  each  has  a  different  purpose. 
One  of  them  remains  exactly  as  it  is  at  the  start,  increasing 


332  BIOLOGY 

by  growth  but  not  changing  in  nature.  Thus  this  substance 
remains  as  germ  plasm,  gp.  The  other  bit  of  the  original  germ 
olasm,  however,  soon  begins  to  develop  into  a  new  individual, 
and  in  distinction  from  the  other  germ  plasm  is  called  soma- 
plasm  (Gr.  soma  =  body  +  plasma  =  substance),  sp.  With 
the  development  of  the  egg  the  dormant  power,  which  this 
somaplasm  possesses,  begins  to  show  itself  in  an  active  form. 
As  a  result  there  appears  a  new  individual;  the  second  gen- 
eration (Fig.  139,  5)  arises  from  this  somaplasm.  The  second 
generation,  in  other  words,  unfolds  the  characters  which  lie 
dormant  in  the  bit  of  germ  plasm  from  which  it  was  derived. 
As  this  individual  develops,  the  other  part  of  the  original  egg, 
which  remains  as  modified  germ  plasm,  finds  lodgment  within 
the  body  of  the  new  individual,  and  thus,  when  the  somaplasm 
has  developed  into  an  adult,  that  adult  contains,  stored  away 
somewhere  in  its  body,  a  bit  of  this  dormant  germ  plasm  of 
the  original  egg.  Since  this  germ  plasm  has  not  changed  its 
nature,  but  has  only  increased  in  amount,  its  nature  is  of  course 
exactly  the  same  as  that  of  the  parent  germ  plasm. 

When  later  the  second  generation  produces  eggs,  some  of 
this  germ  plasm,  which  has  been  stored  away  in  the  body  of 
the  second  generation,  passes  into  each  of  the  eggs;  Fig.  139,  4. 
If  we  admit  that  the  germ  plasm  has  certain  dormant  qualities, 
capable  of  developing  into  an  adult,  it  will  of  course  follow 
that  all  of  the  individuals  produced  from  bits  of  this  germ  plasm 
will  be  alike.  It  is  thus  inevitable  that  the  third  generation 
should  be  exactly  like  the  second,  since  both  the  third  and 
the  second  generations  have  developed  from  two  different 
parts  of  the  same  germ  plasm.  As  long  as  the  process  con- 
tinues, it  is  evident  that  successive  generations  will  be  alike. 
Part  of  the  germ  plasm  at  each  reproduction  is  handed  on 
unchanged  to  the  next  generation;  it  is  retained  by  that  gen- 
eration through  its  life,  and  then  handed  on  again  to  the  next 
generation.  Successive  generations  thus  carry  a  continuous  germ 
plasm.  The  race  is  the  result  of  the  continuous  germ  pla^m; 


THE  ORIGIN  AND  DEVELOPMENT  OF  ORGANISMS     333 

the  individual  is  simply  the  unfolding  of  a  bit  of  it,  the  soma- 
plasm,  which  is  set  aside  to  develop  into  an  individual  for  the 
purpose  of  carrying  for  future  generations  the  germ  plasm  which 
is  to  continue  the  race.  Heredity  is  thus  due  to  the  continuity 
of  the  germ  plasm  from  generation  to  generation. 

It  is  seen  that,  in  accordance  with  this  theory,  heredity  is 
simply  a  name  given  to  a  process  of  handing  on  from  age  to 
age  a  bit  of  marvelous  material,  the  germ  plasm,  small  bits  of 
which  have  the  property  of  developing  into  individuals.  As 
long  as  this  germ  plasm  is  handed  on  unchanged,  it  will  pro- 
duce a  succession  of  generations  identical  with  each  other,  and 
there  will  be  a  conformity  of  type.  It  will  be  seen  thus  that 
the  child  does  not  actually  inherit  anything  from  its  parents; 
the  child  and  parent  are  alike  because  both  develop  characters 
that  are  present  in  the  continuous  germ  plasm. 

Variations  in  Germ  Plasm  are  Inherited. —  It  is  evident  that 
any  modification  of  the  germ  plasm  must  permanently  affect 
the  race.  If  at  any  period  the  germ  plasm  should  be  changed 
so  as  to  produce  in  it  a  new  character,  the  new  character  will 
inevitably  appear,  not  only  in  the  next  generation,  but  in  the 
following  generations.  Characters  which  appear  in  the  germ 
plasm  at  once  become,  therefore,  race  characters,  handed  on  with 
certainty,  unless  something  subsequently  causes  their  disappear- 
ance from  the  germ  plasm. 

Variation  in  the  Individual  not  Inherited. — It  is  equally  evi- 
dent that  any  variation  occurring  in  the  body,  but  not  in  the 
germ  plasm,  will  have  a  very  different  effect  upon  the  race. 
The  individual  is  only  a  trustee  of  the  germ  plasm  which  is 
stored  away  somewhere  in  his  body.  Among  animals,  the  germ 
substance  is  largely  stored  away  in  the  ovaries  and  sperm 
glands;  among  plants  it  may  be  more  distributed,  but  here 
also  it  is  probably  located  in  certain  parts  of  the  plant.  If 
we  admit  that  the  individual  has  nothing  to  do  with  this  germ 
substance  except  handing  it  on  to  the  subsequent  generations, 
it  is  evident  that  no  special  change  which  affects  the  individual 


334  BIOLOGY 

himself  will  be  transmitted  to  the  subsequent  generations.  If 
an  individual  should  sustain  the  loss  of  an  arm,  it  would  affect 
his  own  life,  but  would  have  no  influence  upon  the  germ  sub- 
stance which  he  has  received  from  the  egg,  and  which  he  is 
simply  holding  hi  trust  for  the  next  generation:  his  offspring 
will  not  be  one-armed.  So  with  any  peculiarity  developed 
during  life,  as  the  result  of  life  habits  or  as  the  direct  result 
of  environment.  Characters  which  are  impressed  simply  upon 
the  individual  himself  will  have  no  opportunity  of  being 
transmitted  to  subsequent  generations. 

Congenital  and  Acquired  Characters. — Thus  it  will  be  seen 
that  variations  are  of  two  distinct  types:  (1)  variations  which 
appear  in  the  germ  plasm  and  which  therefore  affect  subsequent 
generations;  (2)  variations  which  appear  hi  the  body  of  the 
individual  and  which  are  not  in  the  germ  plasm,  and  hence 
cannot  affect  subsequent  generations.  These  two  types  of 
variations  have  been  recognized  for  a  long  time,  but  they 
were  never  sharply  distinguished  until  Weismann's  conception 
of  heredity  brought  them  out  hi  such  clear  contrast.  Char- 
acters which  result  from  modification  of  the  germ  plasm,  and 
hence  are  inevitably  transmitted  by  heredity,  to-day  are  com- 
monly called  congenital  characters.  Congenital  variations  are 
fixed  in  the  germ  plasm  and  are  therefore  inevitably  trans- 
mitted by  the  process  of  heredity.-  On  the  other  hand,  char- 
acters which  are  developed  as  the  direct  result  of  the  environ- 
ment, such  as  loss  of  limbs,  or  changes  resulting  from  food  habits, 
climate,  etc.,  are  commonly  known  by  the  name  of  acquired 
characters.  The  term  is  not  a  good  one,  for  all  characters  are 
acquired  at  some  time;  but  this  name  has  been  used  hi  the 
discussion  of  the  last  quarter  of  a  century,  for  such  variations. 
From  what  has  just  been  stated,  it  is  evident  that  acquired 
characters,  if  they  do  not  become  a  part  of  germ  substance, 
will  not  be  repeated  in  subsequent  generations.  Acquired 
characters,  therefore,  which  an  individual  animal  or  plant 
develops  as  the  result  of  the  conditions  of  the  environment  in 


THE  ORIGIN  AND  DEVELOPMENT  OF  ORGANISMS     335 

which  he  lives,  would  affect  his  body  during  life,  but  would  not 
be  expected  to  affect  his  progeny;  and  acquired  characters 
should  not  be  transmitted  by  heredity.  This  conclusion,  quite 
at  variance  with  the  beliefs  of  twenty-five  years  ago,  has  been 
subjected  to  long  and  exhaustive  study,  as  a  result  of  which 
the  belief  in  the  inheritance  of  acquired  characters  has  gradually 
disappeared.  The  conclusion  has  been  vigorously  disputed,  and, 
since  the  advancement  of  Weismann's  theory  of  heredity,  the 
most  active  search  has  been  made  for  proof  or  disproof  of  the 
idea  that  acquired  characters  can  be  inherited.  While  many 
apparent  instances  of  such  inheritance  are  easily  found,  they  all 
prove  illusive  when  carefully  studied,  and  biologists  have  prac- 
tically agreed  that  there  is  no  good  evidence  that  acquired  char- 
acters can  be  transmitted  to  subsequent  generations.  While  the 
possibility  of  the  inheritance  of  acquired  characters  cannot  yet 
be  positively  denied,  it  is  quite  generally  believed  to-day  that 
it  does  not  occur.  This  conclusion  has  far-reaching  results,  for 
it  entirely  changes  our  conception  of  the  relation  of  parent  to 
offspring. 

Heredity  and  the  Union  of  Sex  Cells. — We  are  in  a  position 
now  to  appreciate  a  little  more  fully  the  significance  of  the 
factor  of  the  union  of  sex  cells  in  sexual  reproduction.  Thus 
far  heredity  has  been  spoken  of  as  associated  with  eggs  only. 
A  succession  of  similar  types  in  successive  generations  can  be 
explained  as  due  to  a  division  and  transmission  of  a  continuous 
germ  plasm.  But  the  result  of  such  a  process  would  seem  to 
produce  a  series  of  like  individuals,  without  any  variation  in 
successive  generations.  Successive  generations  are,  however, 
not  alike.  Indeed,  the  development  of  animals  and  plants  is 
dependent  upon  the  fact  that  successive  generations  show  more 
or  less  divergence  from  the  original  type.  It  is  here  that  we 
see  one  reason  for  sexual  reproduction. 

In  Chapter  XII  we  have  noticed  that  the  reallj-  significant 
feature  of  the  union  of  the  egg  and  the  sperm  lies  in  the  fact 
that  each  of  these  reproductive  cells  throws  away  part  of  its 


336  BIOLOGY 

chromatin  in  order  to  make  room  for  a  similar  amount  brought 
to  it  by  the  other  of  the  two  uniting  sex  cells.  If,  as  we  have 
seen  reason  for  believing,  the  chromatin  contains  the  germ 
plasm,  this  process  has  a  most  natural  interpretation.  The 
maturation  of  the  egg  and  the  union  of  sex  cells  bring  about 
a  new  individual  in  which  the  germ  plasm  is  a  mixture  from 
two  individuals  (amphimixis)  (Gr.  amphi  =  together  +  mixis 
=  a  mixing) .  The  result  is  that  the  germ  plasm  of  the  subse- 
quent generations  will  be  different  from  that  which  was  pres- 
ent in  either  of  the  parents  of  the  last  generation,  since  it 
will  be  a  mixture  of  the  two,  and,  if  the  parents  are  in  any 
degree  unlike,  the  mixture  of  their  germ  plasms  will  not  be 
exactly  like  that  of  either.  It  would  be  impossible  for  any 
such  complex  things  as  two  bits  of  chromatin  to  be  mixed 
twice  without  producing  differences  in  the  mixtures.  In  other 
words,  the  following  generation  will  show  variations  from  the 
last.  Since,  however,  this  mixed  germ  plasm  will  be  handed 
on  to  form  the  germ  plasm  of  the  next,  and  all  following  genera- 
tions, it  will  follow  that  the  variations  which  thus  appear  will 
be  handed  on  indefinitely  by  the. process  of  heredity,  and  such 
new  characters  as  appear  from  the  mixture  of  two  germ  plasms 
will  remain  fixed  in  the  race.  With  the  next  reproductive  gen- 
eration this  mixed  germ  plasm  will  again  be  combined  with 
another  mixture  from  another  individual,  and  still  further 
variations  will  appear.  Successive  generations  will  thus  tend 
constantly  to  be  more  or  less  unlike  their  parents.  Sex  union 
of  eggs  and  sperms,  therefore,  appears  to  be  a  device  to  bring 
about  variation  and  divergence  from  type. 

If  this  conclusion  is  correct,  we  should  expect  those  organ- 
isms which  multiply  by  sex  union  to  show  a  greater  amount 
of  variation  than  those  which  multiply  by  the  asexual  process 
of  simple  division;  and  this  appears  to  be  a  fact.  If  a  horti- 
culturist wishes  to  preserve  unchanged  a  type  of  plant  which 
he  has  found,  he  contrives  to  multiply  the  plant  by  the  asex- 
ual method  of  budding,  grafting,  or  cuttings.  As  long  as  this 


THE  ORIGIN  AND  DEVELOPMENT  OF  ORGANISMS     337 

method  is  continued  the  plants  remain  essentially  constant. 
If,  however,  he  wishes  to  obtain  new  types,  he  adopts  the 
method  of  planting  the  seeds.  Seeds,  as  we  have  seen,  come 
from  the  sex  process  of  reproduction,  and  the  offspring  which 
come  from  seeds  show  a  far  greater  tendency  to  variability 
than  those  which  come  from  buds  by  the  asexual  process.  It 
is  the  general  conclusion  from  observation  that  variations  are 
more  common  among  organisms  multiplying  by  the  sexual 
process.  With  this  understanding,  one  purpose  and  function 
of  the  union  of  the  sex  cells  becomes  intelligible. 

DIVERGENCE  FROM  TYPE 

The  term  "divergence  from  type"  is  the  exact  opposite  of 
the  term  "conformity  to  type."  It  is  no  less  evident  that 
animals  and  plants  tend  to  diverge  from  the  race  type  than  it 
is  that  they  conform  to  type.  The  reconciliation  of  these  two 
contradictory  facts  is  that  though,  in  general,  successive  gen- 
erations conform  to  the  type  of  the  race,  the  individuals  show 
more  or  less  variation  from  each  other,  and,  moreover,  the 
whole  race  is  slowly  changing,  so  that  the  type  itself  in  time 
undergoes  modifications. 

Individual  Variations. — An  infinite  number  of  slight  differ- 
ences are  found  between  individuals  of  the  same  species.  This 
fact  is  clear  to  everyone  who  is  at  all  familiar  with  animals. 
In  the  human  race,  it  is  well  recognized  that  no  two  individuals 
are  exactly  alike,  and  the  same  thing  is  equally  true  among  all 
species  of  animals  and  plants.  The  different  individuals  of 
the  same  species  differ  in  size,  color,  habits,  and  in  an  infinite 
number  of  minor  points,  like  the  length  of  legs,  the  length  of 
hair,  the  size  and  shape  of  leaves,  flowers,  etc.  Indeed,  there 
is  no  part  of  an  animal  or  plant  that  does  not  show  more  or 
less  of  such  variation.  It  is  so  evident  that  it  needs  no  further 
discussion.  While  the  different  individuals  conform  to  the 
type  of  their  species  in  general  character,  in  numerous  details 
they  differ  from  each  other  in  almost  endless  fashion. 


338  BIOLOGY 

Race  Variations. — In  addition  to  individual  variations,  the 
whole  species  may  show  a  tendency  to  diverge  from  its  original 
form.  Races  are  either  slowly  or  rapidly  changing  from  their 
previous  condition,  so  that  if  the  members  of  any  race  living 
to-day  are  carefully  compared  with  those  living  in  a  previous 
period  of  the  world's  history,  it  will  be  found  that  the  whole 
race  has  undergone  a  general  change  which  has  affected  all 
members.  Such  race  variation  commonly  occurs  by  what 
is  known  as  divergence.  By  this  is  meant  that  the  descendants 
of  one  type  have,  by  this  race  variation,  diverged  in  several 
directions,  more  or  less  different  from  each  other.  This  is 
explained  by  the  assumption  that  the  descendants  from  any 
animal  remain  neither  exactly  like  their  ancestors  nor  like  each 
other,  and  that  different  lines  of  descent  depart  from  the  origi- 
nal type  in  different  directions.  Examples  of  this  are  numerous, 
but  for  illustrative  purposes  two  well-known  instances  of  such 
divergence  will  be  briefly  mentioned. 

Breeds  of  pigeons. — For  some  centuries,  breeders  of  pigeons 
have  been  very  much  interested  in  improving  different  strains 
of  these  birds,  and  pigeon  fanciers  have  been  careful  to  breed 
together  individuals  showing  characters  that  appeal  to  their 
fancy.  The  result  has  been  that  the  pigeons  have  undergone 
many  profound  changes  from  their  original  type.  The  original 
pigeon,  from  which  all  of  our  domestic  pigeons  came,  is  fairly 
well  known  to  be  essentially  the  same  as  the  rock  pigeon  of 
India,  a  bird  gray-blue  in  color,  with  bars  on  its  breast  and  a 
tendency  to  perch  on  rocks,  but  never  on  trees.  Historical  and 
scientific  evidence  shows  that  all  the  numerous  strains  of 
pigeons  with  which  our  pigeon  fanciers  are  familiar  to-day 
have  been  derived  from  this  bird.  The  tumblers,  the  fan- 
tails,  the  pouters,  and  hosts  of  others,  have  all  been  descended 
from  this  primitive  ancestral  form.  The  differences  between 
these  varieties  are  very  numerous,  including  variations  in 
color,  length  of  bill,  size,  wings,  tails,  and  many  other  points, 

The  differences  between  the  breeds  of  pigeons,  which  have 


THE  ORIGIN  AND  DEVELOPMENT  OF  ORGANISMS     339 

thus  been  produced  artificially,  are  greater  than  differences 
found  among  many  of  the  wild  birds  that  are  regarded  as 
belonging  to  distinct  species.  In  the  case  of  the  pigeons,  it  is 
known  from  historical  evidence  that  these  different  strains  have 
all  come  from  a  common  type  by  methods  of  breeding. 

The  dogs. — Another  example,  perhaps  even  better  known,  is 
that  of  the  breeds  of  dogs.  Dogs  have  been  domesticated  for 
a  period  almost  as  long  as  man  has  been  civilized.  At  the 
present  time  the  variety  of  dogs  is  very  great,  ranging  in  size 
from  the  great  Newfoundland  to  the  tiny  poodle,  and  varying 
in  color,  type  of  hair,  disposition,  and  almost  every  other 
respect.  We  can  hardly  conceive  of  two  animals  being  much 
more  unlike  than  the  tiny  lap-dog  and  the  massive  bloodhound 
or  mastiff,  and  it  is  hardly  possible  to  believe  that  these  ani- 
mals have  all  come  from  the  same  type.  But  the  most  careful 
study  of  the  characters  and  history  of  the  breeds  of  dogs  has 
led  to  the  unquestioned  conclusion  that  all  forms  of  domes- 
tic dogs  with  which  we  are  familiar  belong  to  one  species  of 
animal,  and  all  came  from  the  same  type  far  back  in  history. 
Some  varieties  of  dogs,  like  the  dingo  of  Australia,  belong  possi- 
bly to  a  different  species;  but  all  of  our  common  forms  belong  to 
one  species  and  have  been  derived  from  the  same  fundamental 
stock.  Here,  as  in  the  case  of  pigeons,  the  breeds  have  been  the 
result  of  a  long  series  of  unconscious  breeding  experiments. 
Different  families  of  human  beings  have  had  a  liking  for  certain 
types  of  dogs  and  have  kept  by  them  such  individuals  as  pleased 
their  fancy.  These  have  been  bred  together  and  their  masters 
have  selected  from  the  pups  those  which  most  pleased  them. 
This  process  has  gone  on,  similar  individuals  being  bred  with 
each  other  over  and  over  again,  until  the  whole  race  has  become 
slowly  changed.  Different  types  of  dogs  were  selected  for  differ- 
ent purposes.  The  shepherd  took  a  fancy  to  a  different  type  of 
animal  from  that  which  was  most  desirable  as  a  house  dog.  By 
selecting  the  dogs  who  could  drive  sheep,  or  the  big  dogs,  or  the 
fierce  dogs,  or  the  little  dogs,  etc.,  and  breeding  together  those 


310  BIOLOGY 

nearest  alike,  there  have  been  produced  the  different  types  which 
we  have  in  our  world  to-day.  Recognizing,  however,  that  all 
of  these  types  of  dogs  belong  to  the  same  species  and  must  have 
come  from  a  single  common  type,  the  strains  of  dog  illustrate 
excellently  well  what  is  meant  by  race  divergence. 

Both  of  these  examples  have  been  chosen  from  domestic 
animals.  There  is  no  reason  for  doubting  that  the  same  facts 
may  occur  in  nature  and  that  under  proper  conditions  in  nature 
there  may  be  a  series  of  race  variations  similar  to  those  found 
in  domestication.  Perhaps  divergence  in  nature  is  not  quite 
so  rapid  or  so  extreme  as  it  is  when  controlled  by  the  fancy 
of  the  breeder,  but  the  same  general  facts  hold  true.  In  nature 
as  well  as  under  domestication,  races  are  undergoing  a  constant 
series  of  changes,  sometimes  slow,  and  sometimes  rapid. 

Race  variations  must  be  variations  of  the  germ  plasm. 
Individual  variations,  as  we  have  seen,  will  affect  the  body  of 
the  individual  but  will  not  affect  the  germ  substance.  From 
this  it  follows  that  individual,  acquired  variations  will  not  be 
transmitted  by  heredity  and  will  therefore  have  no  lasting 
effect  on  the  race.  On  the  other  hand,  if  the  race  is  to  undergo 
a  change,  as  we  have  just  seen  that  it  does,  this  must  be  due 
to  modifications  in  the  continuous  germ  substance.  Hence  it 
follows  that  the  only  variations  that  can  continue  in  the  race 
and  can  be  carried  on  for  successive  generations,  are  those  that 
affect  the  germ  material  itself.  Race  variations  are  therefore 
necessarily  germ  variations. 

The  Divergence  from  Centers. — A  little  thought  will  show 
that  the  result  of  divergence  of  the  descendants  of  any  type 
in  different  directions  will,  in  the  end,  produce  extremely  wide 
diversity  among  animals  and  plants.  If  the  descendants  of 
any  animals  diverge  in  two  directions,  and  then  later  their 
descendants  again  diverge  from  each  other,  and  if  this  process 
goes  on  indefinitely,,  it  becomes  evident  that  in  the  course  of  time 
the  descendants  of  the  original  type  will  become  widely  unlike 
«ach  other,  and  will  show  great  variation  from  primitive  forms. 


THE  ORIGIN  AXD  DEVELOPMENT  OF  ORGANISMS     341 

Such  has  been  the  history  of  animals  and  plants.  So  far  as  we 
can  learn  of  that  history,  it  has  always  been  one  of  divergence 
from  common  centers,  the  process  being  repeated  over  and  over 
again  in  successive  ages,  until  finally  there  has  resulted  the 
great  diversity  of  organisms  that  people  the  world  of  to-day. 

At  the  beginning  of  life  in  the  world  there  was,  apparently, 
no  difference  between  animals  and  plants.  We  have  already 
seen  that  some  organisms  so  closely  resemble  both  groups  that 
we  cannot  say  whether  they  are  animals  or  plants.  Possibly 
some  such  organisms  were  the  first  to  inhabit  .the  world.  As 
progress  continued,  however,  the  descendants  of  these  origi- 
nal forms  of  life  diverged  from  each  other  along  two  great 
lines,  one  of  which  acquired  the  habit  of  living  upon  the 
other.  The  original  form  of  life  must  have  been  capable  of 
utilizing  the  mineral  ingredients  hi  nature,  possibly  like  the 
green  plants  of  to-day.  Whether  the  original  organisms  were 
capable  of  carrying  on  photosynthesis  we  do  not  know,  but 
hi  some  way  they  must  have  been  able  to  utilize  minerals. 
However  that  may  be,  their  descendants  diverged  into  two 
groups,  one  group  acquiring  the  green  coloring  matter  and  the 
power  of  utilizing  carbon  dioxid  and  sunlight  and,  by  means 
of  chlorophyll,  building  up  starch,  thus  giving  rise  to  plants. 
The  second  group  of  descendants,  losing  this  power  of  utilizing 
minerals,  and  acquiring  the  power  of  feeding  upon  the  mate- 
rials which  were  manufactured  by  the  first  group,  developed 
into  the  kingdom  of  animals. 

After  plants  and  animals  were  thus  separated  and  each  had 
developed  for  a  time  along  its  own  line,  some  of  the  plants  lost 
their  chlorophyll  and  acquired  the  habit  of  depending  upon 
other  plants  for  food,  thus  becoming  the  Fungi.  As  the  history 
of  the  world  progressed,  each  of  the  two  great  types  thus 
started  continued  to  repeat  the  history  of  divergence.  Age 
after  age  the  descendants  continued  to  separate  into  different 
lines,  until  the  modern  world  was  finally  produced,  with  its 
endless  series  of  different  forms,  all  having  been  derived  from 
common  centers  by  descent  with  divergence. 


CHAPTER  XVIII 


THE  ORIGIN  OF  THE  LIVING  MACHINE;  ADAPTATION; 
THE  FORCES  OF  ORGANIC  EVOLUTION 

ADAPTATION 

Meaning  of  Adaptation. — One  of  the  most  striking  facts  of 
the  organic  world,  resulting  from  heredity  and  variation,  is  the 
adaptation  of  animals  and  plants  to  their  environment.  By 
this  term  is  meant  that  the  parts  of  each  animal  and  plant  are 
so  particularly  fitted  to  the  conditions  of  its  life  that  it  seems  as 
if  they  were  intelligently  fashioned  with  this  end  in  view. 

A  few  illustrations  will  make  the  matter  a  little  clearer.  The 
tree,  with  its  roots  extending  under  ground,  with  its  branches 
growing  into  the  air  and  bearing  the  broadly  expanded  leaf 
surface  for  the  purpose  of  absorbing  air,  is 
evidently  exactly  adapted  for  its  life  in  the 
soil  and  in  the  air.  The  roots  are  mechani- 
cally built  so  that  they  can  push  their  way 
through  the  soil;  the  stems  are  rigid  enough 
to  support  the  heavy  branches,  and  the 
leaves  are  broad  and  thin  and  of  exactly  the 
proper  shape  to  absorb  the  largest  amount  of 
air.  The  wing  of  a  bird  is  an  example  of 
adaptation;  for  its  structure,  its  shape,  the 
lightness  of  its  bones,  its  ability  to  expand  its 
feathers,  the  delicate  manner  in  which  the 
parts  of  the  feathers  are  attached  to  each 
other,  are  all  admirably  adapted  to  an  organ  whose  function 
is  to  support  the  bird  in  the  air.  The  bird's  feet  are  a  beauti- 
ful instance  of  adaptation,  since  wading  birds,  swimming  birds, 
and  scratching  birds  have  feet  plainly  adapted  to  their  peculiar 

342 


FIG.  140.  — THE 

PENGUIN,  A  BIRD 
ADAPTED  FOR  LIFE 
IN  THE  WATER 


ORIGIN  OF  THE  LIVING  MACHINE:  ADAPTATION      343 

habits  of  life;  Figs.  140  to  143.  The  white  fur  of  the  polar 
bear  is  an  adaptation  to  its  life  habits  in  the  north  on  the  ice 
sheets;  for  not  only  does  the  heavy  hair  serve  as  a  warm  covering 


FIG.  141. — THE  FOOT  OF  A  BIRD  ADAPTED  FOR  WADING  IN  MUD 

to  protect  the  animal  from  the  cold,  but  its  color  at  a  distance 
is  hardly  to  be  distinguished  from  the  white  ice,  and  thus  pro- 
tects the  bear  from  observation.  The  marvelous  tongue  of  the 


FIG.  142. — THE  FOOT  OF  A  BIRD       FIG.  143. — THE  FOOT  OF  A  BIRD 

ADAPTED  FOR  SCRATCHING  ADAPTED  FOR  SWIMMING 

butterfly  is  adapted  for  sucking  the  honey  from  flowers.  The 
honey  in  the  flower  is  at  the  bottom  of  the  long  corolla,  and 
unless  the  butterfly  had  this  long  tongue  to  insert  within  the 


344  BIOLOGY 

corolla  and  thus  reach  the  honey,  it  would  not  be  able  to  utilize 
this  food.  Each  butterfly  is  provided  with  a  tongue  sufficiently 
long  to  obtain  the  honey  from  the  particular  kind  of  flower 
upon  which  it  feeds.  The  marvelous  structure  of  the  human 
hand,  with  its  wonderful  mobility,  its  delicate  sensations,  its 
great  power  of  muscle  movement,  is  clearly  adapted  for  use 
as  an  organ  of  prehension,  and  one  might  believe,  as  has  been 
vigorously  argued,  that  it  was  especially  made  by  an  intelli- 
gent designer  for  the  conditions  of  life  in  which  man  lives. 

The  principle  of  adaptation  is  found  everywhere  in  nature,  all 
animals  and  plants  being  more  or  less  adapted  to  their  conditions 
of  life.  Indeed,  perhaps  the  most  characteristic  feature  of  or- 
ganisms is  that  they  are  adapted  to  their  environment,  instead 
of  being  purely  haphazard  in  their  shape  and  structure.  In- 
animate objects,  like  stones,  have  no  special  relation  to  their 
environment,  and  having  been  produced  by  blind  forces,  are 
not  particularly  adapted  to  any  purpose.  In  contrast  to  this, 
all  animals  and  all  plants  show  structure  and  functions  which 
fit  them  for  their  environment.  We  may  almost  regard  this 
feature  of  adaptation  as  the  most  universal  and  striking  char- 
acteristic of  life. 

Origin  of  Adaptation. — How  came  organisms  to  be  thus 
adapted  to  their  environment?  The  explanation  of  adaptation 
which  was  for  a  long  time  regarded  as  satisfactory,  was  that 
each  animal  was  made  by  an  intelligent  Creator,  and  exactly 
fitted  to  the  environment  in  which  it  was  placed.  This  sug- 
gestion was  satisfactory  so  long  as  it  was  believed  that  each 
species  was  an  independent  creation.  Since,  however,  the  idea 
of  special  creation  has  been  replaced  by  the  belief  that  our 
present  species  have  been  derived  from  older  types  by  descent, 
the  problem  of  adaptation  to  their  environment  must  be  given 
a  different  solution.  If  animals  have  diverged  from  common 
centers,  it  follows  that  types  now  inhabiting  different  localities 
must  have  originally  come  from  the  same  place,  and  if  they 
were  originally  adapted  to  one  locality,  they  could  not  be 


ORIGIN  OF  THE  LIVING  MACHINE:  ADAPTATION      345 

especially  adapted  to  the  conditions  of  new  localities.  Hence 
their  adaptation  to  a  new  environment  must  have  been  ac- 
quired during  their  growth,  and  not  by  an  original  special  cre- 
ation. The  question  of  how  the  adaptation  was  produced, 
therefore,  comes  up  with  redoubled  force. 

More  careful  study,  however,  shows  that  animals  are  not 
always  exactly  adapted  to  their  environment.  The  old  idea 
that  each  organism  is  especially  fitted  for  its  environment  is  not 
borne  out  by  facts.  Of  course  living  animals  are  always  in  a 
measure  adapted  to  the  conditions  in  which  they  live,  for  if  they 
were  not  they  would  long  since  have  been  exterminated.  Indeed, 
the  history  of  animals  shows  many  instances  where  poorly 
adapted  animals  have  been  crushed  out  of  existence,  leaving 
alive  only  those  adapted  to  their  environment.  On  the  other 
hand,  many  instances  are  known  where  organisms  living  in 
one  part  of  the  world  to-day  are  not  particularly  adapted 
to  their  habitat,  but  are  really  better  adapted  to  other  parts 
£>f  the  world  if  they  could  only  get  into  new  regions.  It  not 
infrequently  happens  that  organisms  from  one  country  get 
carried  by  accident  to  another,  and  find  the  new  country  far 
better  adapted  to  their  life  than  their  original  home.  For 
example,  when  the  European  hare  was  carried  to  Australia,  it 
found  conditions  far  better  adapted  to  it  than  those  of  its  original 
home  in  Europe,  and  it  multiplied  with  prodigious  rapidity,  be- 
coming far  more  abundant  in  Australia  than  ever  it  was  in 
Europe.  The  English  sparrow,  when  introduced  from  England, 
finding  America  better  adapted  to  its  life  than  England,  multi- 
plied very  rapidly,  and  spread  over  the  country.  Our  fields  in 
the  eastern  states  are  filled  with  the  so-called  white  daisy  (Leu- 
canthemum) .  This  is  a  European  species  which,  when  introduced 
into  this  country,  found  conditions  better  adapted  to  its  needs 
than  in  its  original  home  and  became  far  more  abundant  here 
than  in  its  original  home.  These  three  illustrations  show  that 
although  animals  certainly  must  be  adapted  to  the  conditions 
in  which  they  live  or  be  exterminated,  they  are  not  particularly 


346  BIOLOGY 

made  for  those  localities,  since  in  many  cases  they  are  better 
fitted  for  other  localities  than  their  own  homes.  The  idea  that 
organisms  were  especially  designed  by  creation  to  fit  the  con- 
ditions in  which  they  live  is  thus  disproved. 

Adaptation  the  Result  of  Growth. — The  history  of  organisms 
shows  that  adaptation  to  environment  has  not  come  suddenly, 
but  has  been  the  result  of  slow  development,  brought  about  by 
race  divergence  and  evolution. 

Adaptation  in  the  life  of  the  individual. — When  the  individual 
starts  its  existence  it  is  simply  a  fertilized  egg.  It  is  a  cell,  and 
is  not  especially  adapted  to  any  particular  condition  of  life. 
In  its  development  the  cell  divides  into  many  cells,  and  these 
cells  assume  different  shapes  and  relations.  As  the  organism 
grows,  the  adaptation  to  the  environment  makes  its  appearance. 
In  plants,  the  roots  soon  assume  a  form  which  adapts  them  to 
the  soil,  while  the  leaves  become  fitted  for  the  air;  in  animals, 
some  cells  adapt  themselves  to  functions  of  digestion,  others 
to  the  functions  of  motion,  etc.  In  other  words,  in  the  life  of 
the  individual,  adaptation  is  a  matter  of  slow  growth  and  comes 
step  by  step  as  the  egg  is  gradually  molding  itself,  into  the  form 
of  the  adult.  Concealed  in  this  fertilized  egg  are  marvelous 
powers  which  cause  the  egg  to  develop  into  an  adult,  and  the 
powers  that  cause  the  development  of  the  egg  cause  also  the 
adaptation  of  the  different  parts  to  the  conditions  of  life. 

Adaptation  in  the  race. — There  is  no  doubt  that  a  similar 
history  of  growth  has  brought  about  the  adaptation  of  the  race 
to  environment.  Probably  the  earliest  type  of  the  plant  was  a 
single  cell,  adapted  to  life  in  the  water  but  not  in  the  soil.  As 
the  ages  passed  on  and  plants  reached  the  land,  an  adaptation 
to  this  new  environment  slowly  developed.  The  structures 
which  we  find  in  animals  and  plants  to-day,  which  adapt  them 
to  their  environment,  were  not  of  sudden  origin  in  any  case, 
but  were  the  result  of  a  gradual  change  of  the  older  forms  into 
newer  types,  more  closely  adapted  to  the  new  conditions  of  life. 

As  an  example  of  such  adaptation,  may  be  mentioned  the 


ORIGIN  OF  THE  LIVING  MACHINE:  ADAPTATION      347 

development  of  the  spinal  column  of  the  vertebrates-  during 
the  geological  ages,  which  is  disclosed  by  the  fossils  in  the 
rocks.  When  the  vertebrates  first  appeared,  apparently  they 
had  no  bones,  but  in  their  backs  was  a  rather  stiff  rod  which 
gave  them  rigidity,  this  being  represented  by  the  rod  in  the 
embryo  which  we  have  already  learned  to  speak  of  as  the 
notochord  (see  page  286) .  Following  along  through  the  various 
strata  of  rocks,  which  represent  a  progressive  development  of 
vertebrates,  we  find  that  this  rod  in  time  became  broken  up 
into  short  sections,  a  condition  which  adapted  its  possessor 
very  much  better  to  an  active  life  in  the  water.  The  short 
sections,  which  became  the  vertebrae,  enabled  a  lateral  flex- 
ing motion  of  the  body  which  could  not  be  brought  about  so 
readily  if  there  were  only  a  stiff  supporting  rod  in  the  back. 
This  broken  series  of  bones,  forming  the  vertebral  column, 
thus  adapted  the  animal  to  its  rapid  motion  in  the  water. 
Later,  when  the  vertebrates  emerged  from  the  water  and  as- 
sumed a  life  on  the  land,  the  type  of  vertebras  adapted  to  life 
in  the  water  was  no  longer  fitted  for  the  condition  in  which 
the  animal  now  lived.  The  vertebrae  were  still  retained,  but 
they  acquired  new  connections  with  each  other,  a  greater  solidity 
and  a  greater  rigidity,  so  that  the  spinal  column  could  now 
support  the  body  in  the  air.  Further  development  of  the  land 
animals  into  the  birds  was  characterized  by  a  further  change  in 
the  form  of  the  vertebrae,  which  adapted  the  animal  to  life  in 
the  air,  and,  moreover,  the  vertebrae  were  changed  in  another 
fashion  in  the  mammals  which  lived  on  the  land.  In  all  of  these 
series  of  changes,  from  the  original  unbroken  rod  of  the  back  in 
paleozoic  times,  to  the  complicated  spinal  column  of  the  mam- 
mal, we  see  a  successive  series  of  adaptations.  The  study  of  fos- 
sils has  made  it  possible  to  trace  this  series  of  changes  in  detail, 
and  our  paleontologists  have  quite  accurately  pictured  for  us 
the  succession  of  changes  that  has  produced  this  long  series  of 
race  variations,  bringing  about  an  adaptation  of  the  race,  first 
to  one  condition  of  life  and  then  to  another,  and  finally  ending 


348  BIOLOGY 

in  the  excellently  adapted  internal  skeleton  which  the  higher 
vertebrates  possess  to-day.  All  of  this  can  be  followed  out  in 
the  study  of  fossils,  and  it  represents  only  one  of  the  many 
series  of  evolutionary  changes  which  have  occurred  in  the  history 
of  animals,  adapting  the  race  little  by  little  to  new  conditions, 
or  better  adapting  them  to  older  ones. 

Forces  Producing  Race  Adaptation. — While  biology  has  not 
yet  reached  a  point  where  it  considers  itself  capable  of  explaining- 
all  of  the  marvelous  phenomena  of  adaptation,  some  of  the  laws 
that  have  been  concerned  in  the  production  of  the  phenomena 
are  fairly  well  understood.  A  primary  one  seems  to  be  the  law 
of  natural  selection,  first  exploited  by  Charles  Darwin.  This 
law  and  its  action  will  be  considered  on  a  later  page. 

THE  THEORY  OF  EVOLUTION 

The  divergence  of  animals  and  plants  from  common  centers 
to  produce  the  diversified  world  of  to-day  has  been  generally 
known  under  the  phrase,  the  theory  of  evolution,  or  the  theory 
of  organic  descent.  The  term  "e volution"  has  a  very  much 
wider  application  than  that  which  has  just  been  given  to  it, 
since  in  its  philosophical  import  it  involves  much  more  than  the 
problem  of  the  origin  of  species  of  animals  and  plants.  The 
general  theory  of  evolution  includes  the  conception  of  the  orderly 
development  of  the  whole  universe,  by  a  system  of  natural  law 
and  force,  and  assumes  that  the  origin  of  the  world  from  the 
original  nebulous  mass  has  been,  from  the  beginning,  due  to 
the  unfolding  of  natural  law.  With  the  philosophical  aspects 
of  the  theory  we  are  not  here  concerned;  but  the  phase  of  the 
theory  that  concerns  the  origin  of  modern  animals  and  plants 
is  one  of  the  fundamental  factors  of  modern  biological  thought. 
Indeed,  it  may  be  stated  that  modern  biology  did  not  have  any 
real  existence  until,  under  the  influence  of  the  writings  of  Charles 
Darwin,  the  conception  of  the  origin  of  species  from  common 
types  began  to  be  studied. 

The  idea  which  has  been  expressed  above,  that  the  adaptation 


ORIGIN  OF  THE  LIVING  MACHINE:  ADAPTATION      349 

of  organisms  to  their  environment  has  been  a  matter  of  growth, 
is  the  result  of  the  thought  of  the  last  half-century.  Previous 
to  the  middle  of  the  last  century  it  had  been  assumed  that  or- 
ganisms transmitted  their  characters  so  accurately  to  their  off- 
spring that  they  had  continued  from  the  beginning  unchanged, 
and  that  species  were  immutable.  The  immutability  of  species 
(Lat.  im  =  not  +  mutabilis  =  changing)  had  been  assumed  as 
the  foundation  stone  of  biological  science,  and  all  conceptions 
of  nature  had  been  based  upon  the  idea  that  organisms  breed 
strictly  according  to  their  type,  without  change,  other  than 
slight  fluctuations  back  and  forth  from  a  center,  and  without 
permanent  modification.  The  conception  which  we  have  as- 
sumed above  —  that  not  only  are  all  organisms  constantly 
undergoing  individual  variations,  but  that  races  are  going 
through  a  gradual  series  of  permanent  changes,  resulting  in  the 
appearance  of  new  forms  with  successive  ages  —  was  quite 
revolutionary  in  thought.  The  belief  that  species  were  not 
immutable,  but  were  constantly  being  transformed  into  new 
species  by  the  ordinary  processes  of  descent,  changed  the  whole 
aspect  of  our  attitude  toward  nature.  During  the  fifty  years 
after  this  conception  was  presented  to  the  world  for  discussion, 
it  was  subjected  to  most  hostile  criticism  and  most  bitter  dis- 
pute. The  objections  have  now,  however,  mainly  disappeared, 
and  it  has  become  to-day  one  of  the  accepted  doctrines  of  science 
that  species  are  constantly  undergoing  changes,  and  that  our 
present  species  have  descended  from  older  ones  and  will  in  turn 
develop  into  others.  To  understand  and  appreciate  this  modern 
conception,  it  is  necessary  to  survey  briefly  the  development  of 
the  idea  and  the  fundamental  facts  that  lie  underneath  it.  In 
this  review  we  will  make  reference  only  to  that  phase  of  the 
great  theory  of  evolution  that  has  to  do  with  the  origin  of  modern 
species,  or  to  organic  evolution,  as  it  is  commonly  termed. 

Early  Views. — We  can  trace  a  beginning  of  the  idea  of  evo- 
lution back  to  the  scientists  and  philosophers  before  Christ. 
Aristotle,  nearly  four  centuries  before  Christ,  recognized  in  a 


350  BIOLOGY 

vague  way  the  idea  of  a  gradual  succession  of  higher  and  higher 
forms  of  existence;  and  several  other  early  philosophers  specu- 
lated concerning  the  origin  of  living  things  upon  the  earth  accord- 
ing to  general  processes  of  development.  But  these  earlier 
ideas  were  soon  lost  sight  of  and  it  was  not  until  the  seventeenth 
century  that  any  more  modern  ideas  of  the  development  of 
animals  from  each  other  were  advanced.  During  all  of  these 
centuries,  and  indeed  until  about  the  middle  of  the  nineteenth 
century,  so  far  as  the  subject  was  thought  of  at  all,  the  view 
generally  accepted  was  that  each  different  kind  of  animal  and 
plant  was  an  independent  creation.  This  view  crystallized  into 
the  special  creation  theory  in  the  writings  of  John  Ray  in  1725, 
and  became  the  generally  accepted  view  of  all  scientists.  Dur- 
ing the  seventeenth  and  eighteenth  centuries,  however,  several 
philosophers  expressed,  in  their  writings,  ideas  approximating 
the  belief  that  living  things  do  not  remain  forever  constant,  but 
are  ever  going  through  the  series  of  changes  that  we  have  al- 
ready described  as  race  divergence.  Among  those  whose  writ- 
ings tended  in  this  direction  may  be  mentioned  Kant,  Goethe, 
Leibnitz,  Erasmus  Darwin,  and  others.  With  the  beginning  of 
the  nineteenth  century  these  conceptions  began  to  take  a  more 
definite  shape. 

Lamarck. — Lamarck  was  a  French  naturalist,  living  in  about 
the  beginning  of  the  nineteenth  century,  and  was  well  versed  in 
botany  and  zoology.  He  formulated  a  clearly  defined  doctrine 
of  descent,  and  was  the  first  of  the  modern  scientists  who  had 
any  conception  of  the  theory  of  evolution.  Lamarck  believed 
that  the  fossils  found  in  the  rocks  were  the  ancestors  of 
animals  living  to-day,  and  that  the  organisms  of  the  present 
world  have  been  derived  by  descent  from  those  that  lived  in 
previous  years.  The  changes  that  had  taken  place  in  their 
structure  he  believed  to  have  been  slow  and  gradual,  but  contin- 
uous, and  produced  by  a  variety  of  causes  which  he  specified, 
and  which  have  received  the  name  of  Lamarckian  factors.  The 
chief  of  these  causes  were  the  following: — 


ORIGIN  OF  THE  LIVING  MACHINE:  ADAPTATION      351 

1.  The  direct  effect  of  the  environment  acting  upon  animals  and 
plants,  modifying  them,  generation  after  generation. 

2.  New  physical  needs,  necessitating  new  conditions  of  life; 
these  new  conditions  producing  changes  in  the  animals  them- 
selves. 

3.  Use  and  disuse. — It  is  a  well-known  fact  that  the  use  of 
any  organ  causes  it  to  increase,  and  the  failure  to  use  it  causes 
it  to  decrease  in  size  and  in  efficiency.    Lamarck  supposed  that 
the  arms  of  birds  became  wings  through  continued  use  in  this 
direction,  and  that  the  hind  legs  of  snakes  were  lost  because 
they  were  not  used.    This  has  been  the  most  universally  recog- 
nized of  the  Lamarckian  factors. 

4.  The  transmission  of  these  acquired  characters  to  posterity. 
Lamarck  assumed,  as  everyone  else  assumed  in  his  day,  that 
any  characteristics  possessed  by  an  animal  or  a  plant  might  be 
transferred  to  its  offspring.    Hence  any  of  the  changes  produced 
by  the  environment,  by  new  physical  needs,  or  by  use  and  disuse, 
would  be  transmitted  to  the  offspring,  and,  therefore,  the  next 
generation  would  have  the  body  modified  by  the   habit   and 
environment  in  which  the  first  generation  lived.     This  would 
result  in  a  constant  modification  of  organisms,  producing  evolu- 
tion. 

There  were  certain  other  factors  in  Lamarck's  conception 
which,  though  really  part  of  the  original  theory,  are  not  com- 
monly included  under  the  term  of  Lamarckian  factors.  One 
of  these  was  cross  breeding,  i.  e.,  breeding  together  of  individuals 
of  different  varieties,  or  perhaps  even  of  different  species,  the 
result  being  an  offspring  different  from  either  parent.  A  second 
was  isolation,  a  suggestion  that  certain  individuals  became 
separated  from  the  rest,  and  they  and  their  offspring,  being 
obliged  to  breed  together,  produced  types  in  an  isolated  locality, 
which  developed  along  lines  different  from  those  taken  by  other 
members  of  the  same  species  in  other  parts  of  the  world. 

Although  these  Lamarckian  factors  are  several  in  number, 
it  will  be  seen  that  there  is  one  common  phase.  In  all  it  is 


352  BIOLOGY 

assumed  that  diversities  produced  in  individuals  as  the  result 
of  the  action  of  the  environment,  or  of  their  own  habits,  i.  e., 
acquired  variations,  are  transmitted  to  subsequent  generations, 
and  serve  as  the  basis  of  the  changes  which  produce  race  varia- 
tions and  evolution.  Our  study  of  heredity  has  shown  that  such 
variations,  according  to  our  present  knowledge,  are  almost 
certainly  not  transmitted  to  subsequent  generations.  It  is 
evident  that  the  very  foundation  of  the  Lamarckian  theory  can- 
not stand,  if  the  modern  conception  of  heredity  is  accepted. 

Lamarck's  views  were  not  accepted  in  his  day.  This  was 
partly  because  the  great  French  naturalist,  Cuvier,  one  of  the 
greatest  naturalists  that  ever  lived,  opposed  them  strongly; 
and  partly  because  the  scientific  world  was  not  at  that  time 
ready  to  accept  any  such  natural  explanation  of  the  origin  of 
organisms  as  that  suggested  by  Lamarck.  They  were,  therefore, 
practically  forgotten  for  a  period  of  fifty  years,  during  which 
time  the  idea  that  organisms  had  appeared  by  the  process  of 
descent  had  practically  no  followers,  special  creation  of  each 
species  to  fit  its  environment  being  the  generally  accepted 
view.  A  new  era  of  thought  was  inaugurated  in  the  middle  of 
the  nineteenth  century  by  Chalmers,  Spencer,  and  especially 
in  1859,  by  Charles  Darwin. 

Charles  Darwin. — Charles  Darwin  was  the  grandson  of 
Erasmus  Darwin,  already  mentioned.  In  1859  he  published  a 
book,  the  result  of  twenty  years'  work,  entitled  "The  Origin 
of  Species,"  which  produced  a  revolution  in  thought,  not  only 
in  science  but  also  in  philosophy.  Darwin  accepted  the  idea  of 
the  origin  of  modern  organisms  from  earlier  ones  by  a  process 
of  direct  descent,  recognizing  that  divergence  of  type  from  com- 
mon centers  has  been  the  law  of  historical  development  of  ani- 
mals and  plants.  To  this  extent,  therefore,  Darwin  followed 
Lamarck  and  the  early  speculators  concerning  the  origin  of 
animals.  Darwin's  method  of  explaining  this  descent  was 
totally  different  from  that  of  Lamarck,  and  much  more  in  ac- 
cordance with  facts  that  could  be  demonstrated.  According  to 


ORIGIN  OF  THE  LIVING  MACHINE:  ADAPTATION      353 

Darwin,  the  method  by  which  new  forms  were  produced  was 
by  the  law  of  natural  selection.  Very  briefly  stated,  that  law 
is  as  follows : — 

1.  Overproduction. — All  animals  and  plants  tend  to  multiply 
more  rapidly  than  it  is  possible  for  them  to  continue  to  exist. 
More  offspring  are  produced  by  even  the  slowest  breeding  ani- 
mals and  plants  than  can  possibly  find  sustenance  in  the  world. 

2.  Struggle  for  existence. — As  the  result  of  overproduction, 
the  individuals  that  are  born  are  engaged  in  a  constant  struggle 
with  each  other  for  the  opportunity  to  live.     This  struggle  is 
sometimes  an  active  and  sometimes  a  passive  one;  and  sometimes 
it  is  a  struggle  with  each  other  for  food.   It  is  a  struggle  in  which 
only  the  victors  remain  alive,  the  vanquished  being  exterminated 
without  living  long  enough  to  leave  offspring. 

3.  Variation,  or  diversity. — All  animals  and  plants  show  a 
large  amount  of  diversity  among  themselves,  and,  as  a  result, 
some  must  be  better  fitted  for  the  struggle  for  life  than  others. 

4.  Natural  selection,  or  the  survival  of  the  fittest. — It  is  a  logical 
result  of  the  struggle  for  existence  that  only  those  individuals 
best  fitted  for  the  struggle  will  be  the  ones,  in  the  long  run,  to 
win  in  the  contest.     Hence  the  "fittest"  in  the  long  run  will 
survive,  while  those  less  fitted  to  exist  will  be  exterminated 
in  the  merciless  struggle  for  existence. 

5.  Heredity. — By  the  law  of  heredity,  individuals  transmit 
to  their  offspring  their  own  characters.    Hence  if  one  individual 
survives  the  struggle  for  existence  by  virtue  of  some  special 
characteristic,  it  will  transmit  this  characteristic  to  its  offspring. 
The  offspring  will  inherit  it,  and  in  the  course  of  a  few  genera- 
tions the  only  individuals  left  alive  will  be  those  that  have 
developed  the  favorable  characteristic  in  question,  while  those 
that  did  not  develop  it  will  be  exterminated  by  the  law  of 
natural  selection. 

As  the  result  of  these  five  factors  working  together,  Darwin 
supposed  that  there  would  be  a  constant  accumulation  of  favor- 
able characters,  each  generation  being  to  a  slight  extent  an 


354  BIOLOGY 

advance  over  the  last.  The  struggle  for  existence  and  the  sur- 
vival of  the  fittest  are  repeated  generation  after  generation,  and 
in  each  successive  generation  the  only  members  to  survive  will 
be  those  with  qualities  that  make  them  better  able  to  contend 
in  the  struggle  for  existence  than  their  rivals.  Hence  every 
individual  character  which  gives  its  possessor  any  slight  advan- 
tage over  its  rival  will  be  sufficient  to  enable  its  possessors  to 
survive  the  struggle  for  existence,  by  bringing  about  the  exter- 
mination of  the  less  fortunate  individuals  that  did  not  have  the 
favorable  character  in  question.  This  character  will  be  trans- 
mitted to  subsequent  generations,  when  the  struggle  will  be 
repeated  again,  and  once  more  the  best  characters  of  the 
next  generation  will  be  selected.  As  this  goes  on  without 
cessation  age  after  age,  there  will  be  a  constant  accumula- 
tion of  favorable  characters,  and  thus  the  race  will  in  general 
constantly  advance. 

Natural  Selection  and  Adaptation. — This  law  of  natural 
selection  is  especially  well  fitted  to  explain  the  marvelous  adap- 
tations of  organisms  to  their  environment.  Since  the  different 
members  of  any  species  of  animals  or  plants  are  not  alike,  it 
will  follow  that  at  any  period  in  the  history  of  a  race,  some  indi- 
viduals will  be  more  closely  adapted  to  their  environment  than 
others.  Since  there  is  always  an  overproduction  of  individuals, 
so  that  many  more  are  born  than  can  live,  it  will  follow  that  the 
individuals  best  adapted  to  their  environment  will  be  the  ones 
that  will  survive,  while  those  less  adapted  to  the  conditions  of 
life  will  be  the  ones  to  be  exterminated  in  the  struggle  for  exist- 
ence. Hence  it  will  follow  that  at  the  close  of  any  generation 
the  individuals  left  alive  will  be  those  that  have  the  most  favor- 
able adaptation  to  environment.  These  will  necessarily  be  the 
parents  of  the  following  generations,  and,  by  the  law  of  heredity, 
the  next  generation  will  inherit  the  characteristics  of  these 
parents  and  will  be,  on  the  average,  a  little  better  adapted  to  the 
environment  than  the  last  generation.  If  this  process  is  repeated 
generation  after  generation,  it  will  follow  that  each  generation 


ORIGIN  OF  THE  LIVING  MACHINE:  ADAPTATION      355 

will  be  slightly  better  adapted  than  the  last.  By  an  accumula- 
tion of  the  improvements  which  thus  appear  accidentally,  there 
will  be  developed,  as  the  generations  pass,  a  closer  and  closer 
adaptation  to  conditions.  The  final  result  is  a  better  adaptation 
to  conditions,  and  a  gradual  change  of  type  and  production  of 
new  species. 

Acquired  and  Congenital  Characters  Affecting  Natural 
Selection. — In  the  form  stated  above,  and  as  at  first  conceived 
by  Darwin,  the  characters  which  are  chosen  by  natural  selection, 
and  upon  which  the  advance  of  the  race  is  based,  might  be  either 
acquired  characters,  such  as  those  upon  which  Lamarck  based 
his  theory,  or  they  might  be  congenital  characters,  which  are  in 
the  germ  plasm  and  essentially  due  to  variation  in  the  heredi- 
tary substance.  Darwin  did  not  sharply  separate  these  two 
types  of  variation,  although  he  recognized  them  both.  Dar- 
win thought  that  the  advancement  of  type  was  produced  prima- 
rily by  the  natural  selection  of  such  characters  as  were  born  with 
the  individual,  i.  e.,  congenital  characters.  He  also  believed 
that,  to  a  certain  extent,  acquired  characters,  which  were  pro- 
duced in  the  animal  either  by  the  direct  effect  of  the  environ- 
ment or  by  use  or  disuse,  could  be  transmitted  and  might  thus 
affect  posterity  and  have  an  influence  in  changing  the  type. 
Darwin  did  not  believe,  as  did  Lamarck,  that  these  acquired 
characters  were  the  primary  factors  in  producing  divergence  of 
type,  but  thought  they  might  be  secondary  ones,  the  primary 
factor  being  the  selection  of  most  favorable  congenital  varia- 
tions. 

Weismann. — The  discussion  of  Darwin's  theories  continued 
vigorously  for  a  quarter  of  a  century,  until  his  views  of  descent 
were  quite  generally  accepted,  although  with  various  opinions 
as  to  the  efficiency  of  his  law  of  natural  selection.  In  1884 
appeared  the  essay  L*  Weismann  "On  Heredity,"  which  put  a 
totally  new  aspect  on  the  whole  problem.  His  theory  of  hered- 
ity, already  described,  was  so  simple,  and  so  readily  obtained 
confirmation  by  direct  observation,  that  it  soon  acquired  almost 


356  BIOLOGY 

universal  acceptance.  With  the  acceptance  of  Weismann's 
theory,  it  was  no  longer  possible  to  look  upon  acquired 
characters  as  transmitted  to  posterity.  As  a  result,  the 
Lamarckian  factors  were  of  necessity  thrown  overboard,  since 
they  all  involved  the  inheritance  of  acquired  characters.  It  was 
no  longer  possible  to  believe  that  the  direct  effect  of  the  environ- 
ment upon  the  individual,  or  the  effect  of  the  disuse  of  organs, 
could  have  any  influence  upon  posterity;  and  as  rapidly  as 
Weismann's  theory  of  heredity  received  acceptance  the  so-called 
Lamarckian  factors  were  discarded,  until  to-day  they  are  not 
generally  regarded  as  factors  in  producing  race  variation.  The 
adherents  of  Weismann  have  thought  that  the  only  possible 
factor  left  to  produce  evolution  was  the  natural  selection  of  the 
congenital  variation.  Congenital  variations,  since  they  are  due 
to  variations  in  the  germ  plasm,  will  be  transmitted;  and  the 
natural  selection  of  these  congenital  variations  will  remain  as 
the  great  factor  in  the  development  of  type.  Indeed,  the  fol- 
lowers of  Weismann  took  this  extreme  view  and  held,  and  still 
hold,  that  the  only  factor  which  has  produced  race  evolution 
has  been  the  natural  selection  of  those  characters  which  start 
as  variations  in  the  germ  substance.  But  the  dispute  between 
the  followers  of  Lamarck's  older  views  and  Weismann's  new 
views  has  never  yet  been  positively  settled.  Some  naturalists 
accept  Weismann's  views  in  toto;  others  have  not  regarded  them 
as  sufficiently  well  demonstrated;  while  quite  a  number  of  prom- 
inent biologists,  including  Spencer,  Packard,  Cope,  and  others, 
have  held  to  a  modern  form  of  Lamarck's  views,  believing  that 
in  some  way,  and  under  some  circumstances,  acquired  characters 
might  have  influence  upon  the  offspring  and  therefore  might 
direct  the  line  of  race  divergence.  The  question  has  not  been 
definitely  settled;  but  at  the  present  time  the  balance  of  evidence 
seems  to  be  against  believing  that  acquired  characters  are  trans- 
mitted, and  therefore  against  the  retention  of  any  of  the  so-called 
Lamarckian  factors,  that  are  based  upon  the  direct  action  of  the 
environment  upon  the  individual. 


ORIGIN  OF  THE  LIVING  MACHINE:  ADAPTATION      357 

The  Mutation  Theory. — One  of  the  essential  factors  of  the 
Darwinian  theory  was  that  the  change  of  species  was  produced 
by  the  selection  of  minute  diversities,  such  as  the  slight  differ- 
ences found  among  animals  and  plants  of  the  same  species.  It 
was  argued  by  Darwin  that  in  the  struggle  for  existence,  when 
the  majority  must  be  exterminated  that  the  few  may  live,  even 
the  slightest  differences  in  structure,  shape,  body,  color,  or  habits 
would  be  sufficient  to  determine  the  question  of  life  or  death. 
If  these  slight  differences  could  accumulate,  generation  after 
generation,  they  would  in  time  become  great;  and  thus,  accord- 
ing to  Darwin,  the  great  differences  between  type  were  produced 
by  the  accumulation  and  heaping  up  of  minute  variations.  To 
many  of  the  more  recent  students  of  this  subject  it  has  not 
seemed  plausible  that  such  minute  differences  could  accomplish 
all  that  Darwin  claimed  for  them.  Many  objections  to  Darwin's 
ideas  on  this  line  have  been  expressed,  and  have  finally  found 
voice  in  a  more  recent  conception  of  the  conditions  which  have 
produced  the  evolution  of  the  living  world.  This  new  idea  is  the 
mutation  theory  (Lat.  mutare  =  to  change),  and  is  commonly 
associated  with  the  Dutch  naturalist,  DeVries,  although  a  num- 
ber of  others  have  shared  in  its  origin  and  development.  DeVries 
based  his  views  upon  observations  made  in  a  field  of  primroses, 
where  he  kept  thousands  of  individuals  under  observation. 
As  the  result  of  these  observations,  he  came  to  the  conclusion 
that  new  types  of  plants  are  appearing  constantly  in  nature; 
but  that  they  do  not  arise,  as  Darwin  had  supposed,  by  the  accu- 
mulation of  little  changes  one  generation  after  another,  but 
suddenly,  and,  as  a  rule,  in  single  steps.  In  his  field  of  primroses, 
growing  side  by  side,  he  found  several  distinct  types,  abso- 
lutely different  from  each  other  and  with  no  intermediate 
steps  between  them.  They  came,  not  as  the  result  of  the  ac- 
cumulation of  little  steps,  but  suddenly,  in  a  single  generation. 
Moreover,  by  isolating  and  experimenting  with  them,  he  found 
that  the  new  characters,  which  had  thus  appeared,  bred  true, 
i.  e.y  remained  fixed  m  the  race. 


BIOLOGY 


lack 


From  this  series  of  observations,  extended  in  other  directions 
by  many  other  observers,  has  been  developed  the  theory  of 
mutation.  This  theory  is,  in  essence,  that  new  characters  do 
not,  as  a  rule,  appear  simply  as  slight  diversities  found  between 
different  individuals  of  the  same  species,  but  as  characters  of 
considerable  extent  at  a  single  birth.  New  features  of  the  race 
are  thus  sudden  in  their  origin  instead  of  gradual,  as  had  been 
supposed  by  Darwin  and  also  by  Lamarck.  According  to  this 
theory  there  are  two  types  of  variation  among  organisms:  1. 
Individual  variations,  spoken  of  above  as  the  diversities  which 

are  shown  between 
different  individuals, 
and  which  come  and 
go  in  a  haphazard 
fashion,  having  no 
part  to  play  in  the 
change  of  the  race. 
These  may  be  ac- 
quired characters;  at 
all  events  they  are 
not  impressed  upon 
the  germ  plasm.  2. 
Mutations,  which 
probably  start  with  the  germ  plasm;  Fig.  144.  These  varia- 
tions may  be  large  or  small,  but  whenever  they  appear  they 
are  at  once  fixed  in  the  race.  Inasmuch  as  they  are  part  of 
the  germ  substance,  they  will  be  handed  on  to  the  next  gener- 
ation and  remain,  therefore,  as  a  permanent  inheritance  of  the 
race.  According  to  the  mutation  theory,  these  sudden  large 
changes  have  brought  about  the  race  divergence.  The  theory 
of  mutation,  therefore,  abandons  Darwin's  idea  of  the  accumu- 
lation of  the  minute  diversities,  and  replaces  it  with  the  idea 
that  the  steps  in  evolution  may  be  larger  and  may  be  taken  sud- 
denly. It  is,  of  course,  evident  that  this  new  conception  of  muta- 
tion is  perfectly  consistent  with  Weismann's  view  of  heredity. 


Red 


BY      THE 


FIG.    144.  —  MUTATIONS    SHOWN 
BEETLE  LEPTINOTARSA 

A  and  C  are  mutants  from  the  original  form  B.    The 
actual  differences  are  greater  than  appears  in  these  fig- 
ures because  of  great  differences  in  color. 
(Tower.) 


ORIGIN  OF  THE  LIVING  MACHINE:  ADAPTATION      359 

Mendel's  Law. — Accompanying  the  development  of  the  the- 
ory of  mutation,  there  has  been  brought  prominently  to  view  a 
somewhat  new  view  of  the  laws  of  heredity,  perfectly  consistent 
with  Weismann's  theory,  but  explaining  its  method  of  action. 
Darwin  in  his  discussion  assumed  that  the  offspring  of  two 
parents,  since  it  could  not  be  like  both,  would,  in  general,  be 
halfway  between  the  two.  Even  the  slightest  familiarity  with 
the  laws  of  heredity  is  enough  to  show  that  organisms  inherit 
from  both  parents,  and  it  has  generally  been  assumed  that  they 
inherit,  or  may  inherit,  equally  from  both.  It  is,  however, 
manifestly  untrue  that  the  offspring  is  always  midway  between 
its  father  and  mother,  inheriting  equally  characters  from  each. 
The  laws  of  heredity  are  much  more  complex  than  this,  for  it 
frequently  appears  that  an  organism  inherits  mostly  from  one 
parent,  the  characteristics  of  the  other  hardly- reappearing  in 
the  offspring.  An  attempt  to  bring  some  of  these  facts  into  a 
general  law  has  resulted  in  what  is  called  Mendel's  law  of 
heredity.  Mendel  published  the  result  of  his  work  originally 
in  1866,  but  it  attracted  no  special  attention  for  nearly  forty 
years,  when  it  was  revived  by  modern  students  in  1900.  Since 
that  time  it  has  been  subjected  to  extensive  experiment,  and 
has  produced  results  of  very  great  practical  value  in  controlling 
and  directing  breeding  experiments  with  animals  and  plants. 
Mendel's  law  is  somewhat  complex  and  difficult  to  understand, 
but  the  essential  features  of  it  -are  as  follows : — 

Unit  characters. — It  is  an  assumption  of  Mendel's  law  that, 
in  many  cases  at  least,  different  characters  of  animals  are  unit 
characters.  By  this  term  is  meant  that  those  characteristics 
are  handed  to  the  offspring  as  single  units,  which  are  inherited 
by  the  offspring  in  toto  or  not  inherited  at  all.  They  cannot  be 
halved  or  reduced  in  total  characteristics.  In  other  words,  if 
the  offspring  inherits  one  of  these  unit  characters,  it  inherits  it  in 
full.  Even  though  the  offspring  should  come  from  two  parents, 
one  of  whom  possessed  the  character  in  question,  while  the 
other  did  not,  the  offspring  would  either  inherit  it  as  a  whole 


360  BIOLOGY 

or  not  at  all.  For  example,  two  varieties  of  peas  are  known,  one 
of  which  has  short  pods  and  the  other  long  pods.  If  they  are 
crossed  the  offspring  are  either  short-podded  or  long-podded, 
but  not  midway  between  the  two.  Very  many  other  characters 
have  been  tested  out  experimentally  and  found  in  the  same  way 
to  be  inherited  as  unit  characters. 

Dominant  and  recessive  characters. — Mendel's  law  further 
points  out  that  some  of  these  unit  characters  are  much  more 
likely  to  reappear  in  the  offspring  than  others.  It  frequently 
happens  that  of  two  opposite  characters,  one  is  much  more  likely 
to  appear  in  the  next  generation  than  the  other.  Those  that 
are  most  likely  to  reappear  are  called,  in  this  terminology, 
dominant  (Lat.  dominari  =  to  rule),  while  other  characters  that 
are  more  likely  to  disappear  in  the  first  generation,  are  called 
recessive  (Lat.  recessus  =  receding).  These  recessive  characters, 
even  though  they  do  not  appear  in  the  first  generation  of  off- 
spring, are  not  necessarily  lost.  The  offspring  may  contain 
within  its  body  the  germs  of  these  characters,  but  they  may 
remain  dormant,  not  appearing  at  all  in  the  first  generation. 
In  subsequent  generations  these  recessive  characters  may  re- 
appear; thus  recessive  characters,  which  are  present  in  one  gen- 
eration, may  disappear  in  a  following  generation,  to  reappear 
subsequently  in  the  later  generations. 

Law  of  inheritance. —  The  specially  valuable  contribution  of 
Mendelism  is  the  formulation  of  a  law  in  accordance  with  which 
these  dominant  and  recessive  characters  reappear  in  subsequent 
generations.  That  law  is  briefly  as  follows :  When  we  cross  with 
each  other  two  individuals,  one  of  which  has  a  dominant  char- 
acter, while  the  other  has  its  opposite  as  a  recessive  character, 
all  of  the  offspring  in  the  first  generation  show  the  dominant 
characteristics.  But  although  showing  only  the  dominant 
characters  they  actually  contain  a  mixture  of  dominant  and 
recessive  characters.  This  is  shown  by  the  fact  that  if  these 
individuals  now  are  bred  together,  in  the  next  generation,  which 
we  will  call  the  second  generation,  only  three-fourths  of  the  off- 


ORIGIN  OF  THE  LIVING  MACHINE:  ADAPTATION      361 

spring  will  show  the  dominant  character,  while  one-fourth 
will  show  the  recessive  character.  If  now  the  individuals  show- 
ing this  recessive  character  are  bred  with  each  other,  all  their 
offspring  will  show  the  recessive  character,  the  dominant  char- 
acter having  totally  disappeared  from  them,  never  to  occur 
again  in  any  subsequent  generation.  This  race  is  then  a  pure 
recessive  type,  from  which  all  of  the  dominant  characteristics 
have  been  eliminated.  All  of  the  other  three-fourths  of  the 
second  generation  show  the  dominant  character  only.  But 
tests,  similar  to  the  above,  prove  that  only  one  of  these  is 
purely  dominant.  The  other  two-fourths,  although  in  them 
the  dominant  character  only  is  evident,  are  really  mixed,  con- 
taining both  dominant  and  recessive  characters.  This  is  shown 
by  the  fact  that  if  they  are  crossed,  three-fourths  of  their  off- 
spring will  again  show  the  dominant  character  and  one-fourth 
will  show  the  recessive  character.  This  process  may  then  be  re- 
peated indefinitely. 

An  illustration  may  make  this  clearer.  Among  mice  the  color 
gray  is  dominant,  while  the  color  white  is  recessive.  If  white 
and  gray  mice  are  bred  together,  the  first  generation  of  offspring 
will  be  all  gray.  If  these  gray  animals  are  now  bred  together, 
in  the  second  generation  three-fourths  of  the  offspring  will  be 
gray  but  one  fourth  will  be  white.  If  these  white  animals  are 
bred  together,  their  offspring  will  all  be  white  and  will  continue 
to  breed  white  offspring  indefinitely,  no  gray  mice  ever  subse- 
quently appearing  in  their  progeny.  They  constitute  a  pure 
white  race.  If  the  other  three-fourths,  which  are  gray,  are  bred 
together,  one  of  the  three-fourths  will  continue  indefinitely  to 
produce  gray  offspring,  no  white  ones  appearing.  In  these  the 
white  characteristic  has  been  eliminated  entirely,  and  they  form 
a  pure  gray  race.  But  the  other  two-fourths,  when  bred  to- 
gether, prove  to  contain  both  white  and  gray  characters,  and 
among  their  offspring  one-fourth  will  show  the  white  fur  and  the 
other  three-fourths  the  gray  fur.  If  again  tested  in  the  same 
way,  the  white  animals  will  be  found  to  produce  pure  races  of 


362  BIOLOGY 

white  with  no  mixture  of  gray  fur;  one  of  the  other  fourths  will 
be  found  to  be  pure  gray  races  with  no  mixture  of  white,  and 
the  other  two-fourths  will  again  prove  to  be  a  mixed  race  con- 
taining both  white  and  gray  characters.  This  process  may 
then  go  on  indefinitely. 

The  further  details  of  this  law  are  too  complicated  to  be  fol- 
lowed out  in  this  place,  but  from  the  law  it  is  possible  to  calcu- 
late approximately  how  many  of  the  offspring  at  each  generation 
will  show  recessive,  and  how  many  dominant  characters.  This 
law  has  been  of  great  value  in  directing  breeding  experiments, 
and  breeders  who  are  trying  to  produce  new  varieties  of  animals 
and  plants  find  the  law  extremely  useful  in  controlling  their  ex- 
I  HT iments  toward  definite  ends.  Mendel's  law  has  thus  shown  that 
the  inheritance  by  the  offspring  of  the  characters  of  the  parents 
is  not  a  pure  matter  of  chance,  but  is  controlled  by  definite 
laws.  While  we  do  not  yet  fully  understand  these  laws,  the  fact 
that  some  of  them  have  been  discovered  gives  promise  that  we 
may,  in  time,  be  able  to  control  the  process  of  inheritance  far 
more  accurately  than  hitherto. 

It  is  not  believed  by  those  who  have  worked  on  Mendel's 
law  that  all  characteristics  of  organisms  are  thus  unit  charac- 
ters and  are  transmitted  in  toto  or  not  at  all.  Some  characters 
appear  to  blend,  as  for  example  the  cross  between  the  white 
race  and  the  negro,  the  offspring  of  such  crossing  being  neither 
white  nor  black  but  mulattoes,  a  mixture  midway  between  the 
parents.  Hence  the  color  of  the  human  skin  is  probably  not 
like  the  white  and  gray  color  in  mice,  a  character  transmitted 
by  the  law  of  Mendel.  This  law  of  Mendel  has,  however,  been 
a  great  contribution  to  science  in  showing  that  large  numbers 
of  characters  or  organisms  are  unit  characters,  and  are  trans- 
mitted according  to  definite  laws  that  may  be  clearly  formu- 
lated. 

We  may  say,  in  concluding  the  general  subject,  that  modern 
biological  science  recognizes  the  principle  that  race  divergence 
has  been  the  law  of  life,  and  that  the  evolution  of  modern  types 


ORIGIN  OF  THE  LIVING  MACHINE:  ADAPTATION      363 

from  earlier  ones  by  descent  has  been  the  method  by  which  the 
present  world  was  produced.  Further,  the  laws  formulated  by 
Darwin,  DeVries,  and  Mendel,  together  with  Weismann's  theory 
of  heredity,  all  fit  together  to  explain  the  method  of  this  evolu- 
tion. New  variations  have  appeared  suddenly,  at  least  in  many 
cases,  as  germ  variations  (mutations) ,  and  then  have  been  trans- 
mitted to  the  offspring  as  unit  characters  by  Mendel's  law,  some 
of  the  offspring  receiving  the  new  characters,  while  others  do  not; 
but  if  inherited  they  are  inherited  as  unit  characters.  Next, 
the  law  of  natural  selection  comes  in  and  selects  those  indi- 
viduals which  have  received  useful  mutations.  Selection  then 
"fixes  them"  in  the  race  by  eliminating  individuals  with  char- 
acters less  useful  than  those  possessed  by  the  survivors.  As  a 
result  of  all  these  factors  working  together  the  race  advances. 


CHAPTER  XIX 
CLASSIFICATION  AND  DISTRIBUTION 

EVEN  the  slightest  familiarity  with  organisms  will  disclose 
striking  similarities  between  some  forms  and  great  differences 
between  others.  The  frog  is  clearly  quite  like  the  lizard  and 
much  like  other  vertebrates,  but  very  unlike  the  earthworm. 
These  points  of  likeness  are  the  basis  upon  which  organisms  are 
classified. 


il 


mt  tar 

FIG.  145. —  THE  SKELETON  OP  A  RABBIT 


c,  carpals; 
/,  fibula ; 
fe,  femur; 
h,  humerus; 


il,  ilium; 
is,  ischium; 
me,  metacarpals; 
mt,  metatarsals; 


pu,  pubis; 
tar,  tarsala; 
r,  radius; 
sc,  scapula; 


at,  sternum; 
t,  tibia; 
u,  ulna. 


Homology. — The  likeness  between  organisms  is  of  two  general 
types.  The  first  is  likeness  in  structure,  which  is  called  homology 
(Gr.  homos  =  like  +  logos  =  ratio) .  It  is  frequently  found  that 

364 


CLASSIFICATION  AND  DISTRIBUTION 


365 


animals  which  appear  quite  unlike  are  really  built  upon  the  same 
plan  of  structure,  differing  only  in  the  manner  that  the  plan  is 
carried  out.  For  example,  the  frog  possesses  a  spinal  column 
made  of  vertebrae,  and  two  pairs  of  legs  attached  to  the  body  by 
girdles,  each  containing  a  certain  number  of  bones.  The  rabbit 
(Fig.  145)  has  a  skeleton  based  upon  the  same  type.  It  also 
possesses  a  spinal  column  made  of  vertebra,  with  two  pairs  of 
appendages  attached  by  girdles  to  the  axis  of  the  body;  and  each 
appendage  is  made  up  of  bones  which  can  be  compared,  bone 
by  bone,  with  those  in  the  appendages  of  the  frog.  If  Figure  145 
is  compared  with  Figure  88,  this  similarity  can  be  seen  and  fol- 
lowed out  in  very  close  detail,  nearly  all  of  the  bones  of  the  frog 
being  represented  in  the  skeleton  of  the  rabbit.  This  similarity 
is  found  in  spite  of  the  fact  that  the  two  animals  are  so  unlike 
in  general  appearance  and  in  habits.  One  lives  in  the  water  and 
uses  its  legs  for  swimming  and  hopping;  the  other  lives  on  the 

land,  using  its  legs  for 

support.  But  although 

.£,       ,.«. 

built  for  different  pur- 
poses, the  skeleton  of 
these  two  animals  is 
evidently  based  upon 
the  same  plan.  Figure 
146  shows  another  ex- 
ample of  similarity  in 
structure  representing 
the  hand  of  man,  C, 
and  the  corresponding 
fore  foot  of  four  other 
animals.  Although  the 
hand  of  man  is  used  for  a  totally  different  purpose  from  that 
of  the  fore  legs  of  the  horse,  the  ox,  or  any  of  the  other  animals 
represented,  it  is  evident  that  they  are  built  upon  the  same  plan 
of  structure.  In  each  there  are  a  radius  and  ulna,  and  a  series  of 
wrist  and  finger  bones.  There  are  differences,  it  is  true:  while 


The  skeleton  of  the  hand  of  man,  C;  and  the  fore  feet 
of  a  horse,  A;  a  rhinoceros,  B;  a  pig,  D;  and  an  ox,E. 
r,  radius ;  u,  ulna.  The  other  bones  are  not  named  but 
may  be  easily  compared. 


366 


BIOLOGY 


man  has  five  fingers  represented,  the  others  have  lost  some  of 
these  fingers,  and  one  of  them,  the  horse,  A,  has  left  but  a  single 

finger  with  the  rudiments  of 
two  others.  That  these  other 
fingers  have  been  lost  has  been 
proved  by  the  study  of  the 
paleontology  of  these  animals; 
for  by  tracing  back  their  his- 
tory through  fossils  it  is  found 
that  the  ancestors  of  the  horse 
had  at  first  five  fingers,  with  a 
type  of  hand  similar  to  that  of 
man;  later  they  had  three  and 
finally  only  one  finger.  This 
similarity  in  the  structure  be- 
tween the  frog  and  the  higher 
animals  is  shown  in  other  parts 
of  the  body  besides  the  bone. 
Figure  147  represents  a  section 
through  the  body  of  a  cat,  giv- 
ing a  diagrammatic  represen- 
tation of  the  relation  of  the 
organs  in  the  upper  part  of 
the  body.  This  can  be  com- 
pared directly  with  the  anat- 
omy of  the  same  region  of  the 
body  of  the  frog,  and  while 
there  are  many  differences  in 
detail,  the  general  structure  is 
evidently  the  same.  The  spi- 
nal cord  with  the  brain  is  found 
on  the  dorsal  surface  of  both 
animals;  the  mouth,  nostrils, 
larynx,  lungs,  and  oesophagus  are,  in  essential  features,  iden- 
tical. Thus  it  is  evident  from  these  comparisons  that  the 


di 


FlG.  147. —  A  MEDIAN  VERTICAL 
SECTION  OF  A  CAT,  SHOWING 
DIAGRAMMATICALLY  THE  RELA- 
TIONS OF  THE  ORGANS 


cb,  cerebellum ; 
cr,  cerebrum ; 
hy,  hyoid; 
Ix,  larynx; 
oe,  oesophagus; 


sp,  spinal  cord; 
st,  sternum; 
th,  thyroid  gland : 
tr,  trachea. 


CLASSIFICATION  AND  DISTRIBUTION 


367 


frog,  the  cat,  and  the  rabbit  are  built  upon  the  same  general 
plan,  which  is  carried  out  in  different  ways  in  different  cases. 
In  other  words,  we  have 
in  these  animals  a  like- 
ness of  structure  quite 
independent  of  differen- 
ces in  the  general  pur- 
poses for  which  the  vari- 
ous parts  of  the  body  are 
used.  A  comparison  of 
Figure  97  with  Figure 
148,  representing  the 
eyes,  respectively,  of  the 
frog  and  of  man,  will 
show  that  this  similarity 
is  carried  out  in  the  de- 
tails of  structure,  even 
of  the  smaller  parts.  Al- 
though differing  in  some 
minor  points,  it  will  be 
easy  to  trace  in  the  eye 
of  the  frog  the  same  parts  that  are  present  in  the  human  eye. 
It  is  perfectly  clear  that  these  two  organs  are  based  upon  the 
same  plan  and  are  identically  planned  structures. 

Such  similarities  in  structure  are  not  by  any  means  confined 
to  animals  with  a  bony  skeleton,  but  may  be  found  among  all 
groups  of  animals.  Figure  149  represents  a  worm,  which,  by 
comparison  with  the  figures  of  the  earthworm  in  Chapter  VIII, 
shows  a  similar  structure  in  spite  of  differences  in  detail.  The 
earthworm  bears  at  first  sight  little  resemblance  to  the  worm 
shown  in  Figure  149,  the  latter  having  external  tentacles  and 
gills,  neither  of  which  is  found  in  the  earthworm.  But  it  will 
be  seen  that  both  are  made  up  of  a  series  of  similar  segments, 
and  that  in  general  shape  they  are  the  same.  If  their  internal 
anatomy  is  compared,  both  are  found  to  have  a  similar  alimen- 


FlG.    148. A    VERTICAL    SECTION   OF 

THE   HUMAN   EYE 

0,  aqueous  humor;          m,  muscle; 
ch,  choroid;  r,  retina; 
co,  cornea;                         sc,  sclerotic; 

1,  crystalline  lens;  si,  suspensory  ligament; 

v,  vitreous  humor. 


368 


BIOLOGY 


tary  canal,  similar  circulatory,  nervous,  and  excretory  systems, 
and  all  other  parts  of  their  anatomy  are  essentially  alike.    Such 

a  likeness  in  struc- 
ture is  sometimes 
found  in  quite  unex- 
pected places.  One 
would  hardly  expect, 
for  example,  that  the 
arm  of  man,  the  fore 
leg  of  a  horse,  the 
wing  of  a  bird,  the 
fore  leg  of  a  frog 
and  the  fin  of  a  fish 
would  be  identical 
structures,  since  they 
FIG.  149.— A  SEGMENTED  WORM  RELATED  TO  viuysomuchinshape 

THE    EARTHWORM,     BUT     HAVING    TENTACLES        and      function;     but 

AND  GILLS  they  are  all  found  to 

be  homologous. 

Analogy. — A  second  type  of  likeness  is.  similarity  in  function, 
irrespective  of  structure.  It  not  infrequently  happens  that  dif- 
ferent animals  develop  organs  of  similar  functions  but  of 
totally  different  structure.  In  this  case  they  are  said  to  be 
analogous  (Gr.  ana  =  according  to  +  logos  =  ratio)  but  not 
homologous.  For  example,  the  butterfly  and  the  bird  have 
both  developed  wings  for  flying,  and  their  wings  are  hence  analo- 
gous. They  are  of  similar  shape  and  are  used  mucb  in  the  same 
way;  but  the  wing  of  the  bird  is  made  of  bones,  muscles,  nerves, 
and  feathers,  while  the  wing  of  the  butterfly  has  none  of  these 
parts,  being  simply  an  outgrowth  of  the  skin  containing  air 
tubes.  It  is  not  homologous  with  the  bird's  wing,  in  spite  of 
similarity  in  shape  and  function.  The  wing  of  the  bird  is,  how- 
ever, both  analogous  and  homologous  with  the  wing  of  the  bat, 
since  both  are  used  for  similar  purposes  and  both  are  made  of 
similar  bones  and  muscles,  nerves  and  blood  vessels.  As  another 


CLASSIFICATION  AND  DISTRIBUTION  369 

example  of  analogous  organs,  may  be  mentioned  the  teeth  in 
the  mouth  of  vertebrates  and  the  peculiar  teeth  found  inside 
the  stomach  of  the  lobster.  These  organs  are  both  used  for 
grinding  food;  but  they  are  not  homologous  organs,  since  their 
structure  is  so  different.  The  teeth  are  bony  organs  arising 
from  the  bones  of  the  skull,  which  are  themselves  developed  from 
the  mesoderm  of  the  embryo;  the  teeth  of  the  lobster  are  of 
horny  texture,  and  are  developed  from  the  ectoderm  of  the  em- 
bryo which  is  folded  inward  to  line  the  stomach.  Numerous 
other  examples  of  analogous  organs  might  be  given,  for  it 
frequently  happens  that  different  animals  use  for  the  same  pur- 
pose organs  that  have  quite  a  different  origin  and  structure. 

Explanation  of  Homology  and  Analogy. — Analogous  organs 
sometimes  show  much  similarity,  as  in  the  shape  of  the  wings 
of  the  bird  and  butterfly,  and  sometimes  very  little.  When 
they  do  show  a  likeness  it  is  explained  by  the  fact  that  similar 
necessities  of  life  have  forced  the  development  of  similar  struc- 
ture. For  example,  both  the  vertebrates  and  the  lobster  are 
obliged  to  masticate  their  food,  and  both  have  consequently 
developed  hard  cutting  and  grinding  surfaces  for  the  purpose. 
There  is,  therefore,  some  similarity  in  the  form  of  the  organs; 
but  there  is  no  necessity  for  similarity  in  structure,  and  in  the 
two  cases  different  parts  of  the  body  have  been  utilized  for  the 
purpose. 

The  likeness  between  homologous  organs,  however,  requires 
a  very  different  explanation,  because  here  we  find  a  similarity 
in  structure  in  spite  of  differences  in  function.  We  cannot  explain 
the  similarity  in  structure  by  any  similarity  of  conditions. 
Although  the  wing  of  the  bird  and  the  arm  of  man  are  adapted 
to  wholly  different  functions  and  have  developed  different  shapes 
and  motions,  they  are,  in  spite  of  this  difference,  formed  upon  the 
same  plan,  with  an  identical  structure.  The  explanation  must 
be  something  more  fundamental  than  mere  similarity  in  use. 
Naturalists  to-day  account  for  likeness  in  homologous  organs  by 
the  theory  of  descent,  saying  that  two  animals  with  homologous 


370  BIOLOGY 

organs  owe  their  likeness  to  the  fact  that  they  have  descended 
from  a  common  ancestor  possessing  such  an  organ.  The  bird, 
the  dog,  and  the  monkey  show  homology  in  the  wing,  fore  leg, 
and  arm,  because  they  have  descended  from  a  common  ancestor, 
whose  fore  appendage  possessed  a  certain  series  of  bones  and 
muscles,  and,  therefore,  all  its  descendants  have,  by  inheritance, 
retained  these  same  bones  and  muscles.  The  differences  between 
the  members  in  question  have  been  brought  about  by  the  fact 
that  they  were  used  for  different  purposes,  and  thus  were  slowly 
modified  in  shape,  although  they  still  retained  a  fundamental 
likeness  in  structure. 

CLASSIFICATION  (TAXONOMY) 

Individuals. — As  we  look  upon  nature  to-day,  we  find  only 
individual  organisms,  each  isolated  from  all  others,  and  allied 
only  with  its  parents.  But  the  most  superficial  examination 
shows  that  some  individuals  have  resemblances  to  each  other, 
while  others  are  very  unlike;  and  it  is  evident  that  organisms 
can  be  arranged  in  groups  showing  more  or  less  likeness  to  one 
another.  Such  a  grouping  is  called  classification.  The  general 
plan  of  such  classification  into  groups  is  as  follows: — 

Species. — When  we  find  a  large  number  of  individuals  re- 
sembling one  another  so  closely  as  to  be  practically  identical, 
we  speak  of  them  as  belonging  to  a  single  species.  For  example, 
the  common  dandelion,  which  is  widely  distributed  over  the 
world,  is  made  up  of  countless  numbers  of  individuals;  but  they 
are  essentially  alike,  in  root,  in  stem,  in  leaf,  in  flower.  We 
therefore  speak  of  them  all  as  constituting  a  single  species,  So 
too  is  the  horse  a  distinct  species,  and  the  ass  another.  To  give 
a  definition  of  just  what  is  meant  by  species  is  impossible,  since 
no  one  knows  just  what  is  meant,  and  the  word  perhaps  does 
not  always  have  the  same  meaning.  That  the  individuals  of 
a  species  are  not  always  exactly  alike  is  evident  from  facts 
already  mentioned  concerning  the  great  variations  among  dif- 
ferent pigeons  and  dogs.  Such  great  variations  as  those  pre- 


CLASSIFICATION  AND  DISTRIBUTION  371 

viously  mentioned  among  dogs  are  very  exceptional,  for  as  a 
rule  the  members  of  the  same  species  are  closely  alike. 

Just  what  biologists  mean  by  species,  and  just  what  line  they 
would  draw  to  separate  two  species  from  each  other,  cannot  be 
stated.  It  is  quite  impossible  to  say  how  unlike  two  animals 
must  be  to  constitute  two  species,  since  sometimes,  as  with 
pigeons,  members  of  the  same  species  may  be  very  unlike, 
while  in  other  cases,  as  with  sparrows,  animals  belonging  to 
different  species  are  very  closely  similar.  It  has  been  quite 
common  to  regard  all  animals  that  can  breed  together  and  pro- 
duce fertile  offspring,  as  belonging  to  the  same  species.  But 
this  is  not  an  accurate  definition  of  the  term,  for  there  are  many 
animals,  so  different  from  each  other  that  they  certainly  deserve 
to  be  ranked  as  different  species,  but  which  can  breed  together. 
Nor  can  we  get  any  idea  as  to  the  meaning  of  the  term  "species" 
by  studying  the  number  of  similar  individuals.  Some  species  are 
composed  of  an  immense  number  of  individuals,  as  in  the  case  of 
the  dandelion;  while  other  species  comprise  very  few  animals, 
sometimes  only  one  or  two  having  been  found.  Sometimes,  too, 
the  organisms  belonging  to  the  same  species  show  a  number  of 
sub-groups,  and  the  biologist  calls  them  sub-species,  or  varieties. 
All  of  these  facts  show  that  no  naturalist  can  at  the  present  time 
exactly  define  the  term  "species,"  or  state  definitely  how  species 
may  be  separated  from  each  other.  When  we  recognize  that 
new  types  are  constantly  arising  from  old  ones  by  the  process 
of  divergence,  it  will  be  seen  that  we  could  not  always  expect 
to  draw  sharp  lines  separating  the  new  and  the  old  types  that 
have  arisen  from  a  common  center.  But  although  naturalists 
are  not  able  to  define  the  term  accurately,  or  separate  the  species 
strictly  from  each  other,  species  are  always  recognized  and  form 
the  starting  point  for  classification. 

Genera. — A  little  study  shows  at  once  that  some  species  have 
a  much  greater  resemblance  to  each  other  than  they  do  to  others. 
For  example,  naturalists  recognize  the  domestic  cat  as  constitut- 
ing one  species,  and  the  wild  cat  as  a  second.  But  it  is  quite 


372  BIOLOGY 

clear  that  the  wild  cat  and  domestic  cat  show  greater  resemblances 
to  each  other  than  they  do  to  tigers,  dogs,  or  wolves.  Moreover, 
it  is  evident  to  anyone  in  the  slightest  degree  familiar  with  ani- 
mals, that  lions,  tigers,  leopards,  wild  cats,  and  domestic  cats, 
although  unlike  each  other,  and  recognized  by  naturalists  as 
belonging  to  different  species,  have  many  points  of  resemblance 
to  each  other.  They  have  the  same  general  stealthy  habits, 
the  same  kind  of  toes  and  feet,  and  they  are  much  more  closely 
allied  to  each  other  than  any  one  of  them  is  allied  to  the 
dog  or  the  wolf.  Naturalists,  therefore,  group  all  of  these 
species  together  under  one  group  which  they  call  a  genus  (pi. 
genera). 

In  naming  any  species,  two  names  are  commonly  used,  the 
first  of  which  is  the  name  of  the  genus,  the  second  that  of  the 
species.  For  example,  Felis  is  the  name  given  to  the  whole  genus 
of  cats.  Felis  domestica  is  the  domestic  cat;  Felis  leo  is  the 
lion;  Felis  bengalis,  the  bengal  tiger;  Felis  canadensis,  the  Ca- 
nadian lynx,  etc.  So,  too,  Viola  is  the  genus  name  of  all  the 
violets;  Viola  blanda,  of  the  white  violet;  Viola  cucullata,  of 
the  common  blue  violet,  etc.  If  the  species  should  happen  to 
have  more  than  one  variety  or  sub-species,  a  third  name  may 
sometimes  be  added  to  indicate  the  particular  variety  of  the 
species.  As  a  rule,  however,  two  names  only  are  used. 

Families. — Extending  observation  a  little  farther,  it  becomes 
evident  that  many  genera  show  close  resemblances  which  mark 
them  off  distinctly  from  other  animals.  As  a  result,  naturalists 
group  genera  together  into  a  larger  group,  which  they  call  a 
family.  A  family  sometimes  may  contain  only  a  single  genus; 
it  may  contain  two  or  three  or  a  large  number  of  genera. 

Orders. — In  the  same  way,  families  are  grouped  together  to 
form  larger  groups,  which  are  called  orders.  For  example,  the 
various  cats  already  considered  have  certain  points  in  common 
with  the  dogs,  wolves,  bears,  seals,  and  walruses.  In  all  of  these 
cases  the  teeth  are  especially  adapted  for  cutting  flesh,  and  the 
animals  are  flesh  eaters.  There  are  very  many  genera  among 


CLASSIFICATION  AND  DISTRIBUTION  373 

them,  and  a  number  of  different  families;  but  all  agree  in  the 
living  upon  flesh,  and  all  show  certain  points  of  likeness  in  the 
structure  of  the  feet  and  the  skeleton,  which  place  them  in  a 
group  by  themselves,  distinct  from  animals  that  live  upon 
vegetable  foods.  All  of  these  flesh-eating  animals  are,  therefore, 
grouped  together  into  an  order  called  the  Carnivora. 

Classes. — In  a  similar  way,  different  orders  can  be  arranged 
in  still  larger  groups.  For  example,  although  there  are  many 
points  of  difference  between  the  carnivorous  cat,  the  herbivorous 
buffalo,  the  gnawing  rabbit,  the  flying  bat,  and  the  gigantic 
marine  whale,  still  they  all  agree  in  one  fundamental  character. 
In  all  of  these  orders  the  females  have  mammary  glands  and 
nourish  their  young  by  means  of  milk,  a  characteristic  which 
is  totally  lacking  in  fishes,  reptiles,  and  birds.  It  is  evident, 
therefore,  that  all  of  these  milk-producing  animals  may  prop- 
erly be  classed  together  under  one  head.  Such  a  group  we  then 
know  as  a  class;  in  this  particular  case  we  name  them  the 
Mammalia. 

Phyla. — Extending  our  observations  still  farther,  we  find 
that  all  of  the  animals  mentioned,  together  with  fishes,  reptiles, 
amphibia,  and  birds-,  resemble  each  other  in  having  bones,  which 
none  of  the  rest  of  the  animal  kingdom  possesses.  The  insects, 
clams,  etc.,  never  have  bones,  but  have  other  characteristics  of 
their  own.  It  is  evident,  therefore,  that  all  animals  possessing 
bones  may  be  grouped  together  as  distinct  from  other  types. 
This  produces  a  group  that  we  know  as  a  phylum  or  sub- 
kingdom.  In  this  particular  case  we  name  the  phylum  the 
Vertebrata. 

Kingdoms. — Now  if  we  sweep  our  glance  over  the  whole 
organic  world,  we  find  that  it  is  divided  into  two  groups,  the 
animals  and  the  plants.  These  large  groups  we  call  the  animal 
kingdom  and  the  vegetable  kingdom. 

Thus  it  is  seen  that  the  whole  organic  world  is  divided  into 
kingdoms,  phyla,  classes,  orders,  families,  genera,  and  species. 
Occasionally  we  recognize  intermediate  groups;  for  instance, 


374  BIOLOGY 

between  the  family  and  the  genera  there  are  sometimes  recog- 
nized what  we  call  sub-families,  between  the  classes  and  the 
orders  we  find  sub-classes,  etc. 

THE  SIGNIFICANCE  OF  CLASSIFICATION 

Why  should  there  be  a  classification?  —  As  soon  as  we 
recognize  the  principle  of  divergence  from  type  it  becomes  evi- 
dent that  the  classification  of  animals  has  a  meaning.  <  'l:i>sifi- 
cation  means  history,  and  if  we  could  get  a  perfect  classification 
we  should  have  pictured  the  history  of  organisms.  The  first 
step  in  the  development  of  the  organisms  of  the  world  was  the 
divergence  of  animals  and  plants  from  one  another,  thus  form- 
ing the  two  kingdoms  of  plants  and  animals.  Then  the  process 
was  repeated  in  each  kingdom,  where  there  appeared  a  still 
further  divergence,  a  number  of  different  lines  of  descent  start- 
ing from  common  centers,  giving  rise  to  the  various  sub-king- 
doms. Again  each  of  these  broke  up  into  other  lines  of  descent, 
and  the  smaller  groups  thus  made  their  appearance.  Thus 
types  continued  breaking  up  and  branching  out  in  various 
directions,  giving  rise  to  a  classification  which  may  be  compared 
to  a  tree,  the  trunk  being  the  original  type  of  organisms,  the  vari- 
ous large  branches  representing  the  first  lines  of  divergence  from 
the  original  stock,  while  the  numerous  subordinate  branches 
represent  the  successive  types  that  appeared,  by  the  same  gen- 
eral law.  The  minute  twigs  at  the  end  of  the  branches  are  the 
species  of  to-day,  and  they  are  aH  connected  by  this  line  of 
descent  with  the  original  trunk. 

The  classification  of  animals  is  the  attempt  to  reconstruct 
this  treelike  arrangement  of  organisms  according  to  their  histori- 
cal relationship.  The  members  of  the  same  species  are  supposed 
to  have  had  a  common  ancestor  in  a  fairly  recent  period;  the 
different  species  of  the  same  genus  had  a  common  ancestor  a 
little  farther  back  in  history;  the  different  genera  of  the  same 
family  had  a  still  earlier  common  ancestor;  the  families  of  the 
same  order  had  their  connecting  point  farther  back  still,  and  so 


CLASSIFICATION  AND  DISTRIBUTION  375 

on  through  the  whole  series,  until  we  get  back  to  the  common 
starting  point,  or  the  common  center  from  which  all  animals  and 
plants  diverge.  Classification  is  thus  an  expression  of  history. 
The  following  is  an  outline  of  the  classification  of  animals  and 
plants.  The  classification  accepted  by  science  is  ever  under- 
going changes,  as  a  more  complete  knowledge  of  relations  is 
obtained,  and  the  classification  accepted  to-day  is  different  in 
many  respects  from  that  adopted  a  generation  ago.  In  turn, 
the  classification  used  to-day  will  doubtless  be  modified  by 
future  study,  until  it  becomes  practically  perfect.  But  even 
though  we  recognize  that  it  is  not  yet  perfect,  it  is  quite  necessary 
to  have  such  a  classification  in  order  to  understand  the  living 
world.  It  must  not  be  inferred  that  our  present  classification 
represents  an  accurate  history  of  organisms.  The  classification 
that  biologists  are  aiming  at  is  a  genetic  one,  i.  e.,  one  that  repre- 
sents actual  relationships,  and  to  a  considerable  extent  the  classi- 
fication outlined  below  does  represent  such  relationships.  But 
the  difficulties  of  determining  the  actual  history  of  organisms 
have  been  so  great  as  to  seem  in  some  respects  almost  insur- 
mountable. The  classification  of  organisms  given  to-day  rep- 
resents, therefore,  only  an  attempt  to  express  genetic  relation- 
ships, and  is  recognized  as  being  only  in  part  successful. 

AN  OUTLINE  OF  THE  CLASSIFICATION  OF  THE  LIVING  WORLD 
THE   PLANT   KINGDOM: 

Phylum  I.     THALLOPHYTA:  plants  without  distinction  of  root, 

stem,  or  branch. 
Sub-phylum  1.   Algae:    thallophytes  possessing  chlorophyll: 

including  unicellular  forms,  pond  weeds,  seaweeds,  etc. 
Class     I.   Diatomacece:  the  diatoms  (Fig.  68  A). 
Class    II.   Cyanophycece:  the  blue-green  algae  (Fig.  68  C). 
Class  III.    Chlorophycece:  the  green  algae  (Fig.  30). 
Class  IV.    Phceophycece:  the  brown  algae. 
Class    V.    Rhodophycece:  the  red  algae. 


376  BIOLOGY 

Sub-phylum  2.  Fungi:  thallophytes  without  chlorophyll :  bac- 
teria, yeasts,  molds,  etc. 

Class     I.  Schizomycetes:  the  bacteria  (Fig.  33). 

Class    II.  Saccharomycetes:  the  yeasts  (Fig.  32). 

Class  III.  Phycomycetes:  the  alga-like  fungi  (Fig.  42). 

Class  IV.  Ascomycetes:  the  sac-fungi. 

Class    V.  Basidiomycetes:  the  basidio-fungi  (Fig.  115). 

Phylum  II.     BRYOPHYTA:  the  moss-like  plants. 

Class      I.    Hepaticce:  the  liverworts. 
Class    II.   Mustinece:  the  mosses. 

Phylum  III.     PTERIDOPHYTA:  the  ferns  and  their  allies. 

Class     I.   Filicales:  the  true  ferns  (Fig.  124). 
Class    II.    Equisetales:  the  horse-tails. 
Class  III.    Lycopodiales:  the  club  mosses. 

Phylum  IV.     SPERM ATOPHYTA:  the  seed-bearing  plants. 

Sub-phylum  1.    Gymnospermae :    the  cone-bearing  plants, 

pines,  hemlocks,  etc. 
Sub-phylum  2.   Angiospermse :  flowering  plants. 

Class    I.   Monocotyledons:  endogenous  plants. 

Class  II.    Dicotyledons:  exogenous  plants. 


THE  ANIMAL   KINGDOM: 
Division  I.     PROTOZOA:  unicellular  animals. 

Class  I.  Rhizopoda:  animals  with  naked  protoplasm  and 
pseudopodia  (Fig.  19). 

Class  II.  Infusoria:  animals  with  cilia,  flagella,  or  ten- 
tacles, and  usually  a  mouth  (Fig.  21). 

Class  III.  Sporozoa:  parasitic  animals,  producing  spores 
and  having  a  metamorphosis  (Fig.  25). 


CLASSIFICATION  AND  DISTRIBUTION  377 

Division  II.     METAZOA:    many-celled  animals  with  a  differ- 
entiation of  cells. 

Phylum  I.   PORIFERA:  animals  with  no  distinct  mouth,  but 
many  incurrent  openings :  the  sponges. 

Class    I.   Calcarea:  with  a  skeleton  of  calcareous  spicules. 
Class  II.    Non-calcarea:  with   a   skeleton   of   silicious   or 
horny  spicules,  or  none. 

Phylum  II.   CCELENTERATA:    animals    with    a   mouth,  but 
without  an  anus  and  with  no  body  cavity. 

Class     I.  Hydrozoa  (Fig.  69). 

Class    II.  Syphozoa:  sea-nettles. 

Class  III.  Actinozoa:  corals,  anemones. 

Class  IV.  Ctenophora. 

Phylum  III.   ECHINODERMATA:  radiate  animals,  with  com- 
plete alimentary  canal  and  a  body  cavity. 

Class     I.  Asterioidea:  starfishes. 

Class    II.  Ophiuroidea:  brittle  stars. 

Class  III.  Echinoidea:  sea-urchins. 

Class  IV.  Crinoidea:  sea-lilies. 

Class    V.  Holothuroidea:  sea-cucumbers. 

Phylum  IV.      PLATYHELMINTHES :     flat,    unsegmented 

worms. 

Class     I.  Cestoda:  the  tapeworms. 

Class    II.  Trematoda:  the  flukes. 

Class  III.  Turbellaria:  the  planarians. 

Phylum  V.   NEMATHELMINTHES :    round,    unsegmented 
worms:  round  worms,  threadworms. 

Phylum  VI.   MOLLUSCOIDEA. 

Class    I.    Polyzoa:  sea-mats,  corallines. 
Class  II.   Brachiopoda:  lamp-shells. 


378  BIOLOGY 

Phylum  VII.   ANNULATA:   the  segmented  worms. 
Class     I.   Chcetopoda:  bristle-footed  worms. 

Sub-class  A,  Polychceta;  with  many  bristles  (Fig.  149). 

Sub-class  B,  Oligochceta ;  with  few  bristles  (Fig.  74). 
Class    II.    Hirudinea:  leaches. 
Class  III.   Archiannelida. 
Class  IV.  Gephyrea. 
Class    V.   Choetognatha. 

Phylum  VIII.   MOLLUSCA. 

Class     I.    Pelecypoda  or  Lamellibranchia:  bivalves,  clams 

oysters,  mussels. 

Class    II.  Gasteropoda:  univalves,  snails. 
Class  III.   Amphineura:  many-valved:  chiton. 
Class  IV.   Cephalopoda:    with  long  arms:  squids,  cuttle 

fishes. 

Phylum  IX.   ARTHROPODA:  with  jointed  feet. 

Class     I.  Crustacea:  crabs,  lobsters,  barnacles. 

Class    II.  Onychophora. 

Class  III.  Myriopoda:  millipedes,  centipedes. 

Class  IV.  Hexapoda:  insects. 

Class    V.  Arachnida:  spiders,  scorpions,  etc. 

Phylum  X.   CHORD  ATA:  animals  with  a  notochord. 

Sub-phylum,  Atriozoa :  body  cavity  opening  to  the  exterior. 

Class    I.    Urochorda:  tunicates  or  sea-squirts. 

Class  II.   Cephalochorda:  amphioxus. 
Sub-phylum,  Vertebrate:  animals  with  a  vertebral  column. 

Class.    I.   Pisces:  fishes. 

Class    II.   Amphibia:  frogs,  toads,  salamanders. 

Class  III.    Reptilia:  lizards,  snakes,  turtles,  alligators. 

Class  IV.   Aves:  birds. 

Class    V.   Mammalia:  mammals. 


CLASSIFICATION  AND  DISTRIBUTION  379 

DISTRIBUTION  OF  ANIMALS  IN  SPACE  AND  TIME 

We  have  already  seen  that  while  organisms  are  always 
adapted  to  the  locality  in  which  they  live,  they  are  frequently 
even  better  fitted  for  other  localities,  and  their  presence  in 
any  part  of  the  world  must  be  due  to  other  factors  besides 
fitness.  The  distribution  of  organisms  on  the  earth's  surface 
is  controlled  by  three  fairly  well-known  laws:— 

1 .  The  members  of  a  species  usually  occupy  a  continuous  terri- 
tory.    We  do  not  find  some  members  of  a  species  in  one  locality 
and  others  in  a  distant  region,  without  finding  them  also  in  inter- 
mediate territory.     There  are  some  exceptions  to  this  law,  but 
in  the  vast  majority  of  instances  each  species  occupies  a  continu- 
ous territory  around  a  center  of  origin.      The  territory  occupied 
will  depend  upon  many  factors  of  climate,  for  of  course  the 
habitat  must  be  properly  fitted  to  furnish  the  organism  with 
food,  water,  and  a  proper  temperature. 

2.  All  animals  and  plants  can  multiply  with  a  rapidity  suf- 
ficient to  give  them,  in  a  comparatively  short  time,  enough  off- 
spring to  cover  the  face  of  the  earth.    The  rate  of  multiplication 
of  different  organisms  varies  very  greatly.    The  codfish  may  pro- 
duce 8,000,000  eggs  per  year,  while  the  elephant  produces  only 
a  single  offspring  in  two  years,  and  usually  not  so  frequently 
as  that.     Among  the  lower  animals  and  plants,  the  rate  of 
reproduction  is  sometimes  even  greater  than  the  higher  num- 
ber given  above.     But  even  the  slow  rate  of  the  elephant  is 
sufficient,  if  the  multiplication  were  unchecked,  to  enable  the 
species  to  fill  the  world  in  a  few  years.    The  numerous  offspring 
are  always  endeavoring  to  find  room  for  themselves,  and  food 
to  eat.    For  this  purpose  they  distribute  themselves  as  widely 
as  possible. 

3.  All  organisms  distribute  themselves  from  the  centers,  where 
their  reproduction  is  rapid.     All  organisms,   even  those  that 
seem  stationary,  have  some  method  of  dispersing  themselves 
over  the  earth.    The  means  of  dispersal  are  chiefly  the  follow- 
ing:   1.  By  independent  migration.    This  is  true  of  almost  all 


380  BIOLOGY 

animals,  but  it  is  not  true  of  plants,  which,  as  a  rule,  have  no 
independent  power  of  motion.  2.  By  winds.  Many  plants 
produce  seeds  or  spores  which  can  be  blown  for  long  distances 
by  the  wind,  until  they  land  in  a  favorable  locality,  where 
they  can  develop  into  new  plants.  This  dispersal  by  the  wind 
is  not  so  common  among  animals,  although  some  of  the  lighter 
animals  which  fly,  like  the  insects  and  bats,  may  be  blown 
for  long  distances  by  the  wind.  3.  By  water  currents.  The 
ocean  currents  and  fresh-water  streams  carry  many  animals 
and  plants  long  distances.  The  Gulf  Stream  carries  living 
organisms  across  the  Atlantic  Ocean,  and  a  river  flowing  through 
a  country  may  distribute  seeds  for  hundreds,  and  even  thou- 
sands, of  miles.  4.  Incidental  means.  There  are  various  inci- 
dental methods  by  which  seeds  or  eggs,  or  even  living  animals, 
may  be  distributed.  Wood-boring  insects  may  be  carried  on 
drifting  logs;  seeds  may  be  carried  in  particles  of  mud  clinging 
to  the  feet  of  flying  birds;  living  animals  may  be  carried  for 
long  distances  on  floating  ice;  ships  carry  living  animals  and 
plants  all  over  the  world;  migrating  animals  not  infrequently 
distribute  seeds  of  plants  as  they  move  about  from  place  to 
place,  and  they  may  even  carry  living  eggs  and  some  living 
animals  in  the  same  way. 

By  some  of  these  means  all  organisms  have  an  efficient 
method  of  distribution,  and  tend  to  scatter  themselves  in  all 
directions  from  the  centers,  where  they  are  produced  in  large 
numbers.  Although  the  dispersal  may  be  slow,  in  the  end 
even  the  most  slowly  migrating  animal  or  plant  might  be 
distributed  over  the  face  of  the  earth.  All  organisms  tend 
to  disperse  themselves  until  further  migration  is  checked. 
The  factors  which  check  their  migration  are  spoken  of  as 
barriers. 

Barriers. — The  ocean. — Bodies  of  salt  water  are  effectual 
barriers  against  the  distribution  of  land  animals.  Flying  ani- 
mals cross  small  bodies  of  salt  water,  and  animals  and  plants 
that  are  blown  by  winds  may  be  distributed  over  the  ocean 


CLASSIFICATION  AND  DISTRIBUTION  381 

for  considerable  distances;  but  for  most  land  animals  the  ocean 
is  an  effectual  barrier. 

The  land. — For  marine  animals,  the  land  proves  to  be  an 
effective  barrier.  Although  the  conditions  are  essentially  the 
same  on  both  sides  of  the  isthmus  of  Panama,  the  animals  on 
the  two  sides  of  the  isthmus  are  different,  the  narrow  land 
barrier  being  sufficient  to  prevent  animals  from  crossing  from 
sea  to  sea.  Land  is  also  a  fairly  effectual  barrier  in  preventing 
the  water  animals  of  one  river  system  from  passing  to  another. 
The  inhabitants  of  the  river  may  distribute  themselves  over 
a  wide  territory,  but  they  are  usually  unable  to  pass  from  one 
watershed  to  another,  except  as  they  may  be  carried  by  inci- 
dental means. 

Mountains. — The  high  mountain  ranges  are  perhaps  the 
most  effectual  barriers  of  all.  Practically  no  animal  or  plant 
is  able  to  cross  over  the  higher  mountain  ranges.  Hence  it 
sometimes  happens  that  the  animals  and  plants  upon  the  two 
slopes  of  high  mountains  may  be  quite  different,  even  though 
the  climatic  conditions  on  the  two  sides  are  essentially  the  same. 

Climate. — Each  animal  and  plant  is  able  to  live  only  in  cer- 
tain conditions  of  climate.  Hence  the  climate  of  a  territory  is 
a  determining  factor  in  regulating  its  inhabitants.  In  their 
distribution,  animals  and  plants  are  frequently  completely 
checked  when  they  reach  territories  in  which  the  climate  is 
unadapted  to  them.  This  may  be  the  result  of  several  different 
factors. 

1.  Water. — The  absence  of  water  is  a  most  effectual  barrier 
to  the  distribution  of  either  animals  or  plants.  Deserts  are 
uninhabited  by  any  form  of  life,  since  no  protoplasm  can  exist 
without  water.  Although  most  forms  of  life  need  a  moist 
climate,  some  prefer  one  that  is  moderately  dry  and  cannot 
live  in  moist  territories.  Deserts  and  semi-deserts  will,  there- 
fore, be  barriers  for  the  greater  number  of  animals  and  plants, 
while  moist  climates  will  be  effectual  barriers  for  the  type  of 
organism  which  prefers  a  semi-dry  climate. 


382  BIOLOGY 

2.  Food. — Animals  and  plants  are  limited  to  territories  which 
furnish  the  food  on  which  they  subsist.    A  territory  that  fails 
to  produce  sufficient  food  for  any  given  type  of  animal  will 
prove  an  effectual  barrier. 

3.  Temperature. — Forms  of  life  adapted  to  a  warm  climate 
cannot  live  in  a  cold  climate,  and  vice  versa.    The  temperature 
of  a  territory  is,  therefore,  a  highly  important  factor  in  deter- 
mining its  inhabitants.   Most  animals  living  in  cold  regions  will 
not  pass  over  the  equator,  and  those  adapted  to  the  warm 
equatorial  climate  cannot  distribute  themselves  over  the  colder 
regions. 

Enemies. — Every  animal  and  plant  has  its  special  enemies. 
These  enemies  are  sometimes  in  the  form  of  parasites;  they 
may  be  larger  animals  and  plants,  or  other  organisms  that  are 
contending  for  the  same  food.  The  mutual  rivalries  of  organ- 
isms make  one  of  the  most  complex  problems  of  biology,  and  one 
that  presents  an  endless  puzzle.  The  introduction  of  any  new 
animals  into  an  old  territory  may  produce  unexpected  changes 
in  the  life  of  the  animals  and  plants,  the  newly  arriving  organ- 
isms seizing  the  available  food,  or  destroying  the  life  of  other 
animals  and  plants,  and  giving  rise  to  modifications  in  the 
fauna  and  flora,  which  can  never  be  anticipated  or  predicted. 
The  complexity  of  these  relations  is  indicated  in  a  famous 
example  given  by  Darwin.  The  clover  crop  is  dependent  upon 
the  bumblebees,  which  distribute  its  pollen  and  produce  proper 
fertilization;  the  number  of  bumblebees  is  dependent  upon  the 
number  of  field  mice  who  eat  them;  the  field  mice  in  turn  are 
eaten  by  the  cats;  so  that  in  this  roundabout  way  the  number 
of  cats  in  a  territory  regulates  the  clover  crop. 

Change  of  type  under  new  conditions. — The  distribution  of  any 
particular  species  of  animal  or  plant  is  modified  by  another 
factor  of  a  different  nature.  When  an  animal  migrates  into  a 
new  territory,  and  comes  under  totally  different  conditions  as 
to  food,  climate,  and  enemies,  it  is  very  apt  to  begin  to  change. 
These  variations  from  the  original  type  may,  in  the  new  terri- 


CLASSIFICATION  AND  DISTRIBUTION  383 

tory,  prove  of  special  advantage  rather  than  of  disadvantage, 
and  will  be  preserved,  while  the  original  type  may  be  destroyed. 
In  the  new  locality,  the  species  often  assumes  a  form  quite 
unlike  the  original  type,  and  becomes  so  differentiated  that  the 
descendants  can  hardly  be  recognized  as  belonging  to  the 
original  species.  This  peculiar  feature  is  especially  noticeable 
on  some  of  the  oceanic  islands.  Such  islands  may  be  hundreds 
of  miles  from  the  mainland  and  only  occasionally  visited  by 
accidental  stragglers;  but  they  develop  peculiar  types  of  ani- 
mals and  plants  distinctly  their  own,  although  originally  com- 
ing from  the  mainland.  So  different  do  they  sometimes  be- 
come that  they  can  hardly  be  recognized  as  close  allies  of  the 
mainland  types.  Although  this  change  of  type  in  new  localities 
is  especially  noticeable  on  oceanic  islands,  it  undoubtedly 
occurs  on  the  continental  areas  as  well.  When  a  species 
migrates  into  a  new  territory,  and  is  placed  under  new  condi- 
tions of  food  and  climate,  and  is  in  rivalry  with  new  enemies, 
modifications  of  the  original  type  are  sure  to  develop,  and  in 
the  end  the  form  adopted  is  more  or  less  different  from  that 
of  the  original  immigrant,  which  may  be  limited  to  its  original 
home. 

DISTRIBUTION  OF  ORGANISMS  IN  TIME:  PALEONTOLOGY 

Geology  discloses  the  fact  that  the  earth's  crust  is  made  up 
of  a  series  of  rocks  which  have  been  deposited  during  the  long 
ages  of  the  past;  and  by  the  study  of  these  successive  layers 
of  rock  we  can  learn  various  facts  concerning  the  history  of 
the  world  during  the  time  when  the  different  strata  were  de- 
posited. In  many  of  these  rocks  we  find  remains  of  living  or- 
ganisms, called  fossils,  which  comprised  the  life  of  the  world 
at  various  periods  in  its  earlier  history.  The  study  of  these 
different  fossil  remains  is  known  as  paleontology  (Gr.  palaios 
=  ancient  +  on  =  being  -f-  logos  =  speech),  and  gives  us  an 
outline  history  of  organisms  in  the  past. 

Paleontological  history  at  best  is  very  incomplete,  since  it 


384  BIOLOGY 

is  only  under  special  conditions  that  the  body  of  an  animal 
or  plant  becomes  imbedded  in  the  rocks  and  preserved  in  the 
form  of  a  fossil.  Incomplete  as  it  is,  paleontology  has  shown 
us  many  illuminating  facts  concerning  the  earlier  life  of  the 
world.  It  has  shown  that  life  has  been  in  existence  on  the 
earth  for  many  millions  of  years,  although  we  have  no  means 
of  determining,  even  approximately,  how  many.  It  has  taught 
that  during  this  long  series  of  ages  there  has  been  a  constant 
succession  of  living  things,  one  type  after  another  making  its 
appearance  and  giving  place  to  other  types.  The  animals  and 
plants  living  to-day  represent  only  the  last  step  in  this  long 
series,  nearly  all  of  the  species  existing  at  the  present  time 
being  of  recent  origin,  some  having  been  in  existence  only  a 
few  thousands,  or  perhaps  even  a  few  hundreds  of  years,  al- 
though some  of  our  present  forms  may  extend  back  for  hundreds 
of  thousands  of  years  in  the  past.  The  immediate  predecessors 
of  our  present  species  were  organisms  much  like  them,  and  from 
them  the  present  forms  have  doubtless  been  descended;  and 
preceding  these  were  others,  still  more  remote  in  time  and  more 
unlike  the  present  ones  in  structure,  representing  still  earlier 
ancestral  forms. 

The  general  history  of  any  series  of  types  has  been  approxi- 
mately as  follows:  Appearing  in  a  certain  part  of  the  world, 
a  group  of  animals  has  dispersed  itself  more  or  less  over  the 
face  of  the  earth,  becoming  numerous  in  species  and  giving 
rise  to  a  variety  of  subordinate  types.  The  development  has 
commonly  gone  on  until  a  climax  has  been  reached,  after  which 
the  particular  type  has  perhaps  remained  constant  for  a  time, 
but  eventually  declined  toward  its  final  extermination.  As 
it  disappeared,  its  place  was  taken  by  some  other  type,  better 
adapted  to  the  new  conditions  of  the  changing  world.  So  the 
progress  has  gone  on  age  after  age,  type  after  type  appearing, 
developing,  culminating,  and  then  declining  and  disappearing. 

One  general  law  in  this  long  progress  is  manifest.  During 
the  whole  sories  of  ages  there  has  been  a  general  progress  of 


CLASSIFICATION  AND  DISTRIBUTION  385 

type  from  lower  to  higher  forms.  The  first  organisms,  appear- 
ing in  the  oldest  rocks,  were  simple  forms  of  low  structure, 
while  the  highest  forms  of  organisms  appeared  in  the  most 
recent  ages.  While  the  progress  has  not  been  uniformly  con- 
stant, the  general  trend  has  always  been  upward.  The  inverte- 
brates, which  contain  the  lower  animals,  appeared  and  cul- 
minated first,  while  the  vertebrates  appeared  later.  Among 
the  vertebrates  the  fishes  appeared  in  the  earlier  rocks,  the 
amphibia  came  next,  reptiles  and  birds  followed,  and  finally 
the  highest  group,  the  mammals,  appeared  last,  with  man  at 
the  extreme  end  of  the  series.  It  is  true  that  in  this  long  suc- 
cession of  ages,  some  forms  of  organisms  have  degenerated, 
becoming  simpler  and  finally  disappearing,  while  others  have 
remained  constant  for  immensely  long  periods  of  time  without 
any  apparent  change.  But  the  general  tendency  of  the  whole 
history  has  been  one  of  progress  from  a  low  form  to  a  higher, 
from  the  simple  to  the  complex;  and  the  living  world  to-day 
represents  the  culmination  of  a  long  period  of  progress  from  the 
earliest  times.  This  progress,  as  disclosed  by  the  fossils  buried 
in  rocks,  is,  in  a  very  general  way,  parallel  to  the  progress  of 
the  individual  animal  as  it  develops  from  the  egg,  through  the 
series  of  changes  which  we  have  learned  to  call  embryology. 
The  parallel  between  embryology  and  paleontological  history 
has  been  one  of  the  striking  discoveries  of  biological  study, 
and  has  been  one  of  the  great  factors  in  the  disclosure  of  the 
unity  of  the  living  world  during  these  long  ages.  All  the  facts 
to-day  assure  us  that  there  have  been  uniform  laws  and  forces 
extending  through  the  whole  series  of  living  organisms,  from 
the  earliest  geological  ages  to  the  present,  and  from  PROTOZOA 
to  MAN. 


GLOSSARY-INDEX 

In  this  index  all  defined  words  are  printed  in  black-faced  type;  words  to  which  only 
page  reference  is  given  are  in  roman  type.  In  addition  to  the  words  used  in  the  text, 
definitions  are  given  for  some  of  the  more  common  biological  terms.  These  may  be  recog- 
nized by  their  lack  of  page  references. 

abdomen. —  The  ventral  part  of  the  body  below  the  ribs,  175. 

abdominal  vein. —  A  vein  in  the  frog,  passing  over  the  abdominal  wall  in 
the  middle  line,  191. 

abducens  (Lat.  ab  =  from  -f-  ducere  =  to  lead). — A  cerebral  nerve  supplying 
the  eye  muscles,  194. 

abiogenesis  (Gr.  a  =  without  +  bios  =  life  -f-  genesis  =  birth) . — See  spon- 
taneous generation. 

aboral  (Lat.  ab  =  away  from  +  os  =  mouth) . —  Opposite  to  or  away  from 
the  mouth. 

absorption  of  food,  205,  306. 

accretion. —  Growth  by  addition  of  layers  on  the  outside,  4. 

accompanying  cells,  106. 

acellular. —  See  syncytium. 

acetabulum,  182. 

achromatin  (Gr.  a  =  without  +  chroma  =  color). — The  part  of  a  cell  that 
does  not  absorb  coloring  matter,  33. 

acquired  characters. —  Characters  first  developed  in  the  body  rather  than 
in  the  germ  plasm,  327;  inheritance  of,  333,  334,  355. 

activity,  2,  3. 

adaptation,  meaning  of,  342;  origin  of,  344,  354. 

adipose  (Lat.  adeps  =  fat). —  Fatty  tissue. 

adnate  (Lat.  adnatus  =  grown  to). —  United  to. 

adrenals. —  Ductless  glands  lying  on  the  kidneys. 

aerial  (Gr.  aer  =  air). —  Pertaining  to  the  ah*. 

aerial  hyphae. —  Branches  of  a  mycelium  growing  upward  into  the  ah*  and 
developing  spores,  98. 

aerobic. —  Growing  only  in  the  presence  of  oxygen. 

afferent  fibers   (Lat.   ad  =  to  +  ferre  =  to  bear). —  Fibers  carrying  im- 
pulses toward  the  brain,  172,  212. 

albumen. —  A  proteid,  illustrated  by  the  white  of  an  egg,  8,  22. 

alcohol,  79. 

alimentary  canal. —  The  digestive  tract,  157,  185. 

alimentation  (Lat.  alimentum  =  food). —  The  process  of  food-getting,  138. 

alveolus. —  Expanded  sacs  at  the  ends  of  ducts,  as  in  the  glands  or  lungs. 
See  page  312. 

387 


388  BIOLOGY 

alternation  of  generations,  in  animals,  277,  in  plants,  269,  273. 

amitosis  (Gr.  a  =  without  +  mitos  =  thread). —  Cell  division  without 
karyokinesis,  89. 

Amoeba,  description  of,  52. 

amoeboid  (Gr.  amoeba  -\-  -oid). —  Resembling  Amoeba,  especially  as  regards 
movements  by  the  protrusion  of  pseudopodia,  218. 

amorphous  (Gr.  a  =  without  -+-  morphe  =  form).  —  Without  regular 
shape. 

amphiaster  (Gr.  amphi  =  around  4-  aster  =  star). —  The  double  star  formed 
in  karyokinesis,  86. 

amphibious  (Gr.  amphi  =  around  +  bios  =  life). —  Capable  of  living  either 
in  the  air  or  in  the  water. 

amphimixis  (Gr.  amphi  =  around  +  mixis  =  a  mixing). —  A  name  applied 
to  the  mixture  of  germ  substance  in  sexual  union,  336. 

amylopsin. —  A  ferment  found  in  the  pancreatic  juice,  that  converts  starch 
into  sugar,  306. 

amylolytic. — Capable  of  converting  starch  into  sugar. 

anabolism. —  The  building  up  of  chemical  substances  from  simpler  ones, 
139,  225,  299. 

anaerobic  (Gr.  an  =  without  -f  aerobic). —  Growing  only  in  the  absence 
of  oxygen. 

anaesthetics  (Gr.  an  =  not  -\-aisthesis  =  feeling). —  Drugs  that  dc.-n<>v 
consciousness. 

analogy  (Gr.  ana  =  according  to  -f  logos  =  a  ratio). —  Likeness  in  func- 
tion, 368. 

analysis  (Gr.  ana  =  up  +  luein  =  to  loose). —  The  reduction  of  a  com- 
pound to  its  component  parts,  234. 

anaphase. —  The  third  stage  of  karyokinesis,  87,. 

anatomy  (Gr.  ana  =  up  -f  temnein  =  to  cut). —  The  study  of  the  grosser 
structure  of  organisms,  19. 

anchylosis  (Gr.  angkylos  =  crooked). —  The  growing  together  of  bones, 

animalculae. —  Microscopic  organisms  commonly  found  in  water,  52. 

animal  functions. — -Those  distinctive  of  animals,  i.  e.,  nervous  and  muscu- 
lar, 217. 

animals  and  plants,  differences  between,  217,  224. 

annuals. —  Plants  which  live  a  single  season  only. 

Annulata  (Gr.  anulus  =  a  ring). —  A  group  of  animals  with  the  body 
divided  into  rings,  157,  378. 

Anopheles,  71,  72. 

antennae.  —  Elongated  appendages  with  sensory  functions  occurring  on  the 
head  of  certain  animals. 

anterior. —  Pertaining  to  the  head,  155. 


GLOSSARY-INDEX  389 

anterior  root. —  The  anterior  branch  of  a  spinal  nerve,  carrying  impulses 

from  the  center,  194. 
anther  (Gr.  anthos  =  a  flower) . —  The  sac  on  the  end  of  a  stamen  bearing 

pollen,  119. 
antheridia  (Gr.  antheros  =  flowery). —  The  organs  in  certain  plants  that 

produce  the  sperms,  271. 

anthropoid  (Gr.  anthropos  =  man). —  Resembling  man. 
anus. —  The  posterior  opening  of  the  digestive  tract;  the  vent,  157. 
aorta. —  The  large  main  artery  which  carries  blood  to  the  lower  part  of  the 

body,  188. 

aperture. —  An  opening, 
apetalous  (Gr.  a  =  without  -+-  petalon  =  a  leaf). —  Flowers  without  petals, 

119. 

appendages. — •  Elongated  projections  from  organisms,  with  special  func- 
tions; like  legs,  tentacles,  etc.,  175. 
appendix  vermiformis. —  A  small,  blind  sac  attached  to  the  end  of  the  large 

intestines. 
APPERT,  14. 
aqueous  humor. —  The  transparent  liquid  between  the  cornea  and  the  lens 

of  the  eye,  197. 
arachnoid  membrane.  —  The  membrane  covering  the  brain,  between  the 

dura  mater  and  the  pia  mater. 
arborizations  (Lat.  arbor  =  a  tree).  —  The  fine  branches   in  which  many 

nerve    fibers    sometimes   terminate,   chiefly   at   their    central    ends, 

170. 
archegonia  (Gr.  arche  =  first  +  gonos  =  race).  —  The  organs   in  certain 

plants  that  produce  small  eggs,  271. 

arteries. —  Blood  vessels  carrying  blood  from  the  heart,  190. 
articular  (Lat.  articulus  =  a  joint). —  Pertaining  to  the  joints,  177. 
ascent  of  sap. — •  The  flow  of  liquids  from  the  roots  to  the  leaves,  126. 
Ascomycetes,  99. 

ascospores. —  Spores  produced  in  asci,  79,  99. 
ascus  (Lat.  ascos  =  a  sac). —  A  sac  holding  a  definite  number  of  spores, 

79,  99. 

asexual  reproduction,  238,  243,  262;  distribution  of,  265. 
asexual  stage  (Gr.  a=  without  +  sexual). —  The  stage  in  a  metamorphosis 

in  which  reproduction  is  by  an  asexual  method. 
Aspergillus,  97,  102. 

asphyxia  (Gr.  a  =  without  +  sphyzein  =  to  throb). —  Suffocation. 
assimilation  (Lat.  assimulare  =  to  make  like). —  The  power  of  converting 

nourishment  into  the  substance  of  the  body,  possessed  by  all  living 

things,  44,  62. 
astragalus,  182. 


390  BIOLOGY 

atrophy  (Gr.  a  =  without  +  trephein  =  to  nourish). —  To  decrease  in  size 
as  the  result  of  disuse,  or  from  other  causes. 

auditory. —  Pertaining  to  hearing,  104. 

auricles  (Lat.  auris  =  ear). —  The  chambers  in  the  heart  that  receive 
venous  blood,  188. 

automatic  activity. —  Actions  started  by  the  organisms  and  not  brought 
about  by  any  external  stimulus:  spontaneity,  3. 

autophytes  (Gr.  autos  =  self  +  phyton  =  plant). —  Plants  which  subsist 
upon  minerals  and  gases,  which  they  utilize  through  the  agency  of  sun- 
light, 226. 

available  energy,  298. 

avidity  for  water,  127. 

axial. —  Pertaining  to  the  axis. 

axial  skeleton. —  The  skull  and  spinal  column,  with  the  ribs  and  sternum, 
177. 

axil. —  The  angle  above  the  attachment  of  a  leaf. 

axis  cylinder. —  See  axon. 

axon. —  The  process  from  a  neuron  extending  outward  and  becoming  the 
axis  cylinder  of  a  nerve  fiber,  170. 

bacillus. —  A  motile,  rod-shaped  bacterium. 

bacteria. —  A  group  of  extremely  minute  plants,  the  simplest  und  sm;ilh>st 
known  organisms,  26,  80,  232,  235. 

ball-and-socket  joints,  185. 

barriers. —  Factors  that  check  the  distribution  of  organisms,  380. 

basioccipital,  180. 

bast. —  The  fibers  of  the  phloem. 

bees,  parthenogenesis  in,  246. 

bell  animalcule. —  Same  as  Vorticella. 

biennials  (Lat.  bi-  =  twice  -f  annus  =  year). —  Plants  that  live  two  years, 
growing  the  first  year  and  fruiting  the  second. 

bilaterally  symmetrical. —  Having  the  two  sides  strictly  counterparts  of 
each  other,  155. 

bile.—  The  secretion  of  the  liver;  also  called  gall,  186. 

biogenetic  law  (Gr.  bios  =  life  +  genesis  =  creation). —  The  law  that  em- 
bryology tends  to  repeat  past  history,  290. 

biological  sciences,  classification  of,  18. 

bladder,  199. 

blade  of  leaf,  114. 

blastula. —  A  hollow-sphere  stage  of  the  developing  egg. 

blend. —  To  mix,  as  when  the  offspring  shows  characters  midway  between 
those  of  its  parents,  362. 

blights,  232. 


GLOSSARY-INDEX  391 

blood,  191. 

blood  vessels,  158. 

body  cavity. —  The  cavity  between  the  intestine  and  the  body  wall;  also 

called  the  coelom,  157. 
body  wall. —  The  muscular  walls  which  lie  outside  the  body  cavity;  the 

outer  wall  of  the  body,  143,  157,  166. 
bone,  27,  176. 

brachial  (Lat.  brachium  =  the  arm). —  Pertaining  to  the  arm,  190. 
bract. —  A  leaf  in  the  axil  of  which  a  flower  is  developed, 
brain. —  The  enlarged  front  end  of  the  nervous  system  in  vertebrates,  192: 

sometimes  applied  to  the  cerebral  ganglia  of  invertebrates,  162. 
branchiae.—  Gills,  288. 
branchial  openings. —  Openings  in  the  neck,  through  which  water  may  pass 

for  respiration,  285. 
bread  raising,  80. 
breeding  season. —  The  season  of  the  year  during  which  reproduction 

occurs,  214. 

bronchi. —  The  larger  branches  of  the  trachea  leading  to  the  lungs, 
buccal. —  Pertaining  to  the  mouth,  185,  284. 
budding. —  Reproduction  by  the  formation  of  buds  which  may  become 

detached;  gemmation,  78,  146,  239. 
bulbus  arteriosus. —  The  large  arterial  trunk  arising  from  the  ventricle  of 

the  frog,  before  it  breaks  up  into  branches,  188;  also  called  truncus 

arteriosus. 

bundle. —  A  cluster  of  elongated  cells,  104. 
butterfly,  72,  289. 
calcaneum,  182. 

calciferous  (Lat.  calx  =  lime  +  ferre  =  to  bear).  —  Lime-producing,  169. 
callosities  (Lat.  callum  =  a  thick  skin). —  Thickenings  of  the  skin, 
calyx. —  The  outer  row  of  leaves  of  a  flower,  usually  green,  119. 
cambium  (Lat.  cambire  =  to  exchange). —  The  layer  of  active,  growing  cells 

inside  the  bark  and  outside  the  woody  layers  in  exogenous  plants,  105, 

108. 

cane  sugar,  9. 

capillaries  (Lat.  capillus  =  a  hair). —  The  microscopic  blood  vessels  be- 
tween the  ends  of  the  arteries  and  the  beginning  of  the  veins,  190,  207. 
capillarity,  127. 

carbohydrates. —  Starches,  sugars,  and  celluloses,  9,  10. 
Carchesium,  91. 

cardiac  (Gr.  cardia  =  heart). —  Pertaining  to  the  heart, 
carnivorous   (Lat.  caro  (carnis)  =  flesh  +  vorare  =  to  eat). —  Feeding  on 

flesh,  224. 


392  BIOLOGY 

carotids. —  The  two  large  arteries  on  the  sides  of  the  neck  carrying  blood 

to  the  head,  190. 
carpals,  182. 
carpels  (Gr.  carpos  =  fruit). —  The  separate  parts  (leaves)  of  which  a  pistil 

is  composed,  120. 
cartilage. — A  hard  material,  softer  than  bone,  which  forms  part  of  the 

skeleton,  27,  35. 

cartilage  bones. —  Bones  first  formed  as  cartilage,  181. 
casein. —  A  proteid  present  in  milk  and  constituting  the  curd,  8. 
castor  bean,  103. 
cat,  366. 

caudal. —  Pertaining  to  the  tail, 
cell. —  One  of  the  simple  units  of  which  living  things  are  composed,  20, 

37. 

cell  doctrine,  38. 

cell  sap. —  A  clear  liquid  inside  of  plant  cells,  30. 
cellulose. —  A  material  related  to  starch  and  forming  the  cell  wall  of  many 

plant  cells;  the  basis  of  paper,  cotton,  etc.,  35. 
cell  wall. —  The  covering  on  the  outside  of  the  cell,  not  present  in  all  cells. 

29,  34. 

central  nervous  system,  162,  192. 
centrosome  (Gr.  centron  =  center  +  soma  =  body). —  A  small  body  lying 

near  the  nucleus  in  animal  cells  and  apparently  the  center  of  active 

forces,  29,  34. 
centrosphere,  34. 

centrum. —  The  large  central  disk  of  bone  in  a  vertebra,  177. 
cephalic  (Gr.  kephale  =  head). —  Pertaining  to  the  head, 
cerebellum. —  The  larger  of  the  two  divisions  of  the  hind-brain,  193. 
cerebral  ganglia. —  The  large  ganglia  in  the  head  of  an  animal,  usually  two 

in  number,  162. 

cerebral  hemispheres. —  The  anterior  and  largest  part  of  the  brain  of  ver- 
tebrates, 193. 
cerebrospinal  axis. —  The  central  nervous  system  of  vertebrates,  composed 

of  brain  and  spinal  cord,  192. 
chemical  composition  of  living  things,  5,  7,  42. 
chemical  compounds. —  Substances  made  of  two  or  more  chemical  elements 

joined  in  chemical  union. 

chemotropism  (Eng.  chemesa  =  chemical  4-  Gr.  trope  —  a  turning). —  Reac- 
tion to  chemical  stimuli,  57. 

chiasma. —  The  crossing  of  the  optic  nerves  in  the  brain. 
Chilomonas,  73. 
chitin. —  A  horny  material,  like  that  of  which  insects'  wings  are  made. 


GLOSSARY-INDEX  393 

chlorogogen  cells.—  The  cells  which  fill  the  typhlosole  and  cover  the  intes- 
tine in  the  earthworm,  169. 

chlorophyll  (Gr.  chloros  =  green  +  phyllon  =  leaf). —  The  green  coloring 
material  in  plants  which  enables  them  to  carry  on  photosynthesis,  93, 
117,  131,  218. 

chlorophyll  bodies,  37. 

chloroplasts  (Gr.  chloros  =  green  -f-  plastos  =  molded). —  Cells  which  pro- 
duce  chlorophyll,  117. 

Chordata  (Gr.  chorde  =  a  string). — Animals  possessing  a  notochord,  includ- 
ing all  Vertebrata,  378. 

choroid  (Gr.  chorion  =  skin). —  The  pigment-holding  layer  of  the  eyeball, 
inside  the  sclerotic  coat,  196. 

choroid  coat,  of  the  eye,  196. 

choroid  plexus. —  A  membrane  full  of  blood  vessels  covering  certain  cavities 
in  the  brain,  193. 

chromatin  (Gr.  chroma  =  color). —  The  material  in  the  nucleus  holding  the 
characteristic  features  of  cell  life  and  concerned  in  inheritance,  33,  259. 

chromatophore  (Gr.  chroma  =  color  +  pherein  =  to  bear). —  A  pigment- 
bearing  cell. 

ohromidial  units,  50. 

chromogenic. —  Pigment-producing. 

chromosomes  (Gr.  chroma  =  color  +  soma  =  body). —  The  threads  of 
chromatin  formed  preliminary  to  cell  division;  the  number  is  constant 
in  each  species  of  organism,  85. 

chyle. —  The  completely  digested  food  in  the  intestine. 

cilia  (Lat.  cilium  =  an  eyelash). —  Vibratile  processes  of  protoplasm  from 
the  free  surface  of  cells,  60,  218. 

circulation,  138,  206,  309;  of  earthworm,  158;  of  frog,  187,  205;  of  Hydra, 
146. 

Cladonia,  229. 

class. —  A  group  of  closely  related  orders,  373. 

classification  of  organisms,  370,  375;  significance  of,  374. 

clavicle,  182. 

cleavage. —  The  division  of  the  egg  into  cells,  281. 

clitellum  (Lat.  clitellce  =  a  saddle). —  A  band  of  swollen  segments  in  the 
earthworm,  between  the  28th  and  35th  segments,  157. 

cloacal  aperture  (Lat.  cloaca  =  sewer). — The  common  opening  of  the  intes- 
tine and  the  urogenital  organs,  175,  186. 

cloacal  chamber,  284. 

cnidoblast  (Gr.  cnide  =  thistle  +  blastos  =  a  sprout). — The  ectodermal  cells 
in  Ccelenterata  which  produce  the  nematocysts. 

cnidocil,  144. 


394  BIOLOGY 

coagulate. —  To  change  into  a  curd-like  mass,  22. 

coccus. —  A  spherical  bacterium. 

cocoon. —  The  t  >ationary  stage  in  the  life  of  a  butterfly,  72;  the  egg  case 

of  an  earthworm,  165. 
Coelenterata  (Gr.  kottos  =  hollow  +  enteron  =  intestine) . —  Animals  with 

pnly  a  single  cavity  and  no  body  cavity,  including  Hydra  and  its  allies, 

377. 
coeliac  axis.  —  The  arterial  trunk  from  the  dorsal  aorta,  supplying    the 

viscera,  190. 

coelom  (Gr.  koilos  =  hollow). —  Same  as  body  cavity,  157. 
Coelomata  (Gr.  koilos  =  hollow) .  —  Animals  with  a  body  cavity  including 

all  animals  above  Ccelenterata. 

coelomic  fluid.—  The  fluid  filling  the  body  cavity,  158,  160. 
coenocyte    (Gr.    koinos  =  common  +  cytos  —  a    cell). —  A    protoplasmic 

mass  containing  several  nuclei,  but  without  division  into  cells;  same  as 

syncytium. 
cold-blooded. —  A  term  applied  to  animals  whose  blood  is  of  essentially  the 

same  temperature  as  the  surrounding  medium, 
colloidal.—  A  term  applied  to  substances  which  will  not  dialyze  through 

membranes,  307. 
colony. —  A  group  of  connected  individuals,  usually  arising  from  one  by 

asexual  budding,  73,  92. 
columella. —  A  rod  connecting  the  tympanic  membrane  with  the  inner  ear 

in  the  fiog,  197. 
commensalism  (Lat.  cum  =  together  +  mensa  =  table). —  An  association 

of  two  organisms  in  which  neither  is  benefited  nor  injured,  230. 
commissures  (Lat.  committere  =  to  join). —  Nerve  cords  connecting  ganglia, 

162. 

communal. —  Living  in  communities. 

compound  pistil. —  A  pistil  made  of  several  fused  carpels,  120. 
conductility. —  The  power  possessed  by  protoplasm  of  transferring  impulses 

from  one  end  to  the  other,  43. 
condyies. —  The  smooth  protuberances  by  which  the  skull  is  attached  to 

the  first  vertebra,  181. 

conformity  to  type. —  The  appearance  of  like  individuals  in  successive  gen- 
erations, 328. 
congenital  characters   (Lat.  con  =  with  +  genitus  =  born). —  Characters 

that  are  fixed  in  the  germ  plasm,  327,  334. 
conidia  (Gr  conis  =  dust). —  Spores  produced  by  constriction  on  the  ends 

of  threads  rather  than  in  a  sporangium,  98. 
conjugation   (Lat.  com  =  together  +  jugare  =  to  join). —  The  union  of 

two  similar  cells  in  reproduction,  65,  94,  247,  262. 


GLOSSARY-INDEX  395 

connective  tissue. —  Material,  usually  fibrous,  which  connects  the  various 

parts  of  an  animal, 
consciousness,  5,  213. 

conservation  of  energy,  294;  applied  to  organisms,  303. 
constructive  processes,  139,  225,  299. 
contagious. —  Having  the  character  of  passing  readily  from  person  to 

person. 

continuity  of  germ  plasm,  329. 
contractile  vacuole,  55,  62. 
contractility,  54. 

convolutions. —  Folds  of  the  surface  of  the  brain, 
coordination  (Lat.   con-  =  with  +  ordinare  =  to  arrange). — •The   orderly 

control  of  the  various  functions  so  that  they  act  in  harmony;  the  control 

of  many  muscles  so  that  they  act  toward  a  definite  end,  140,  162,  211; 

in  plants,  137. 
copulation. —  The  union  of  the  sexes  for  the  transferring  of  sperms  to  the 

eggs,  165,  215. 
copulatory  organs. —  Organs   used   for  bringing  the  sex  cells  together, 

256. 

coracoid,  182. 
cork,  38. 
cornea  (Lat.  corneus  =  horny). —  The  front,  transparent  covering  of  the 

eye,  196. 

corolla. —  The  second  row  of  leaves  in  a  flower,  usually  colored,  119. 
coronary  arteries. —  Arteries  supplying  the  heart, 
corpuscles. —  Any  small  bodies,  but  chiefly  applied  to  floating  cells  in  the 

blood,  192,  205. 

correlation  of  forces. —  Same  as  transformation  of  energy, 
cortex  (Lat.  cortex  =  bark). —  The  layers  of  cells  inside  the  epidermis  and 

outside  the  cambium  of  a  young  stem;  the  outer  layers  of  any  organ, 

as  the  cerebral  cortex,  104,  112. 
cotyledons. —  The  leaves  of  a  plant  in  the  seed,  123. 
cranial  nerves. —  The  nerves  arising  from  the  brain,  194. 
cranium  (Gr.  cranion  =  skull). —  That  part  of  the  skull  that  holds  the  brain, 

180. 
crop. —  An  expanded  chamber  of  the  digestive  tract  for  storing  hastily 

swallowed  food,  158. 
cross  fertilization. —  Fertilization  of  eggs  from  one  individual,  with  sperms 

from  another,  165,  267. 
crus,  182. 
cryptogams  (Gr.  cryptos  =  concealed  +  gamos  =  marriage).—  Plants  which 

do  not  produce  flowers,  103. 


396  BIOLOGY 

crystalline. —  Applied  to  substances  that  will  dialyze  through  membranes, 
307 

crystalline  lens. —  The  lens  in  the  eyeball  which  focuses  light  on  the  retina, 
197. 

Culex,  72. 

cutaneous  (Lat.  cutis  —  skin). —  Pertaining  to  the  skin,  191,  209. 

cuticle  (Lat.  cutis  =  skin). —  A  thin,  structureless  membrane  forming  on 
the  outside  of  the  epidermis,  62,  166. 

cyclical  changes. —  Changes  which  pass  through  a  cycle  but  eventually 
return  to  the  starting  point,  5. 

cyst. —  A  hard  shell  which  is  sometimes  secreted  around  organisms  in  a  dor- 
mant condition;  any  sac  with  a  wall,  developing  abnormally  in  the 
body,  59,  74,  241. 

cytoblastema,  38. 

cytoplasm  (Gr.  cytos  =  cell  +  plasma  =  substance). —  The  liquid  part  of 
the  protoplasm  outside  the  nucleus,  32,  49. 

dandelion,  370. 

DARWIN,  352. 

death,  3,  153. 

decay. —  Decomposition  changes  produced  by  bacteria  in  the  presence  of 
air;  more  complete  than  putrefaction,  81. 

deciduous. —  A  term  applied  to  plants  that  shed  their  leaves  in  the  fall; 
also  to  mammals  that  shed  the  placenta  at  birth. 

decomposition. —  The  chemical  destruction  of  molecules.  In  biology  the 
disintegration  of  organic  substances,  usually  produced  by  bacteria  or 
allied  organisms. 

degeneration,  233. 

dehiscence  (Lat.  dehiscere  =  to'  open). —  The  opening  of  an  organ  to  dis- 
charge its  contents,  123. 

dendrites  (Gr.  dendron  =  tree). —  The  branching  processes  arising  from 
neurons,  170. 

denitrification. — The  reduction  of  nitrates  to  nitrites  or  simpler  compounds. 

dentine. —  The  inner,  softer  part  of  the  teeth. 

depressant. —  Having  the  power  of  reducing  activity. 

dermis. —  The  inner  layer  of  the  skin,  176. 

descent  theory. —  The  theory  that  all  organisms  are  genetically  inter- 
connected: evolution,  348. 

dessication  (Lat.  dessicare  =  to  dry  up). —  Drying,  57. 

destructive  processes,  139,  300. 

deutoplasm  (Gr.  deuteros  =  second  +  plasma  =  substance). —  The  food 
yolk  in  the  egg,  249. 

DE  VRIES,  357. 


GLOSSARY-INDEX  397 

dextrose. —  A  form  of  sugar  found  in  fruits;  glucose,  9. 

dialysis. —  See  osmosis. 

diaphragm. — A  muscular  membrane  separating  the  chest  from  the  abdomen. 

diastatic. —  Capable  of  turning  starch  into  sugar. 

diastole  (Gr.  diastole  =  an  expansion). —  The  period  in  each  beat  when  the 
heart  is  relaxed,  188. 

Diatoms,  136,  219. 

dichotomous  (Gr.  dicha  =  in  two  +  temnein  =  to  cut). —  Branching  by 
regular  division  into  pah's. 

differentiate. —  To  become  unlike;  usually  applied  to  parts  originally  similar 
but  which  acquire  different  structure  and  function,  95,  283. 

digestion. — •  A  series  of  changes  in  the  chemical  and  physical  nature  of  the 
food  which  renders  it  capable  of  absorption,  204,  305. 

digestive  cells,  145. 

digestive  juices. — •  The  secretions  which  render  the  food  capable  of  absorp- 
tion, 55,  62,  204. 

digits. —  Fingers  and  toes. 

digitigrade. —  Walking  on  the  tips  of  the  fingers  and  toes. 

dimorphism  (Gr.  di-  =  twice  -f-  morphe  =  form). — -Showing  two  distinct 
forms. 

dioecious  (Gr.  di-  =  twice  +  oikos  =  house) . —  Having  the  sexes  in  different 
plants. 

diphtheria,  231,  232. 

direct  development. — •  Development  without  a  metamorphosis,  290. 

disease  germs,  82. 

disintegrate. —  To  break  to  pieces,  4. 

dispersal. —  The  power  of  organisms  to  distribute  themselves  from  centers, 
379. 

dissepiment. —  See  septum. 

distal. —  Farthest  from  the  main  body. 

distribution,  in  space,  379;  in  time,  383. 

divergence.—-  The  appearance  of  two  or  more  lines  of  descent  from  a  com- 
mon center,  337,  339. 

diversities. —  The  slight  differences  found  among  individuals  of  the  same 
species. 

diverticulum. —  Any  sac-like  outgrowth. 

dogs,  origin  of,  339. 

dominant  characters  (Lat.  dominari  =  to  rule). —  Those  which  appear  most 
prominently  in  the  first  generation  after  the  crossing  of  races,  360. 

dorsal. —  Pertaining  to  the  back,  155. 

drones. —  Male  bees. 

Drosera,  223. 


398  BIOLOGY 

ductless  glands. —  Gland-like  structures  without  ducts,  pouring  their  secre- 
tions into  the  blood. 

ducts. —  The  large  spiral  or  otherwise  marked  cells  in  the  fibrovascular 
bundles;  vessels,  106.  In  animals  the  tubes  that  carry  the  secretions 
of  glands  to  the  exterior,  105. 

duodenum. —  The  first  loop  of  the  intestine  below  the  stomach,  186. 

dura  mater  (Lat.  durus  =  hard  +  mater  =  mother). —  A  tough  membrane 
on  the  outside  of  the  brain,  194. 

ears,  197. 

earthworm,  155;  physiology  of,  216. 

ecdysis. —  The  shedding  of  the  skin. 

ecology  (Gr.  oikos  =  house  +  logos  =  discourse). —  The  study  of  the  mu- 
tual relations  of  animals  to  each  other  and  to  their  environment, 
20. 

ectoderm  (Gr.  ectos  =  outside  +  derma  =  skin). —  The  outer  layer  of  cells 
of  animals,  141,  283. 

ectoparasites. —  Parasites  living  on  the  outer  surface  of  their  host,  230. 

ectoplasm. —  The  outer  layer  of  protoplasm  in  Protozoa,  54,  61. 

efferent  nerve  fibers  (Lat.  ex  =  from  -}-ferre  =  to  bear). —  Fibers  carrying 
impulses  away  from  the  brain,  172,  212. 

egg. —  Same  as  ovum,  267. 

egg  sac,  163. 

electropism  (Gr.  electron  =  amber  +  trope  =  a  turning). —  The  power  of  re- 
acting to  electricity,  58. 

elements. —  The  ultimate  varieties  into  which  substances  can  be  chemically 
analyzed,  5. 

embiyo  (Gr.  embryon  =  an  embryo). — The  young  organism  in  the  early 
stages  of  development,  19. 

embryology. —  The  study  of  the  development  of  the  egg  into  an  adult,  19; 
of  the  frog,  280. 

embryo  sac. —  A  name  formerly  given  to  the  macrospore  of  a  flowering 
plant,  122,  273. 

emulsion. —  Finely  divided  droplets  of  one  liquid  (usually  oil)  floating  in 
another  liquid,  23. 

enamel. —  The  hard,  outer  covering  of  the  teeth. 

encyst. —  To  inclose  in  a  cyst,  74,  241. 

endoderm  (Gr.  endon  =  within  +  derma  =  skin). —  The  inner  layer  of  cells 
of  animals,  lining  the  digestive  tract,  143,  145,  283. 

endodermis. —  A  layer  of  cells  within  the  cortex  and  next  to  the  wood  in 
the  roots  of  plants,  113. 

endogenous  stem  (Gr.  endon  =  within  +  genes  =  a  producing). —  Stems 
in  which  the  fibrovascular  bundles  are  irregularly  arranged,  with  no 
cambium,  wood  ring,  or  bark,  112. 


GLOSSARY-INDEX  399 

endoparasites. —  Parasites  living  within  the  body  of  their  host,  231. 
endoplasm     (Gr.     endon  =  within  +  plasma  =  substance). —  The     inner 

layers  of  protoplasm  in  the  Protozoan  cell,  54,  62. 
end  organs. — Special  organs  at  the  ends  of  the  nerves,  211;  peripheral  and 

central  end  organs  are  recognized, 
enemies,  relation  of  animals  to,  382. 
energy,  292;  stored  by  plants,  299. 
English  sparrow,  345. 
enteron. —  The  alimentary  canal,  158. 
entire. — Of  a  leaf  margin,  without  indentations, 
environment. —  The  surroundings  which  influence  organisms,  351. 
enzymes. —  Substances  secreted  by  organisms  and  having  powers  of  fer- 
mentation; unorganized  ferments,  306. 

epiblast. —  A  name  applied  "to  the  ectodermal  layer  of  the  embryo, 
epidermis  (Gr.  epi  =  upon  -f-  derma  =  skin). —  The  outer  layers  of  cells 

of  any  organism,  104,  115,  167,  176. 
epiglottis   (Gr.  epi  =  upon  -f  glottis  =  glottis) . —  An  elastic  lid  covering 

the  glottis,  which  prevents  food  from  passing  into  the  windpipe, 
epiphysis. — -Same  as  pineal  gland,  193. 
epithelio-muscle  cells. —  Cells  of  the  ectoderm  of  Hydra,  with  contractile 

fibers  extending  from  their  base,  143. 

epithelium  (Gr.  epi  =  upon  +  thele  =  nipple) . —  Cell  layers  covering  sur- 
faces or  lining  canals  or  cavities,  169. 
equatorial  plate. —  The  flattened  mass  of  chromosomes  formed  between 

two  centrosomes,  86. 
erythrocytes  (Gr.  erythros  =  red  +  cytos). — •  The   red  corpuscles   of   the 

blood,  192,  205. 
Eudorina,  263. 
Euglena,  75,  76,  217. 
Eustachian  tubes. —  The  tubes  leading  from  the  throat  to  the  middle  ear, 

186,  197. 
eversion  (Lat.  e  =  out  -f  vertere  =  to  turn). —  The  process  of  turning  a 

part  inside  out. 
evolution. —  The  theory  that  traces  the  origin  of  the  present  world  from  the 

past  as  the  result  of  the  unfolding  of  natural  law,  348. 
excreta,  139. 
excretions. —  Waste  products  of  metabolism  eliminated  by  glands,  56,  139, 

210. 
excretory  system.—  56,  62,  139;  of  earthworm,  161;  of  frog,  199;  of  Hydra, 

151;  of  plants,  225. 
exoccipital  bones,  180. 


400  BIOLOGY 

exogenous  stems  (Gr.  exo  —  without  +  genes  =  a  producing) . —  Stems 
with  a  cambium  layer  separating  a  bark  from  a  wood  ring,  109,  112. 

extensors. —  The  muscles  that  straighten  the  appendages  at  the  joints,  211. 

eye,  196;  of  human  being  and  of  frog  compared,  367. 

eyespot. — A  colored  spot  found  in  unicellular  organisms,  sensitive  to  light, 
76. 

facial  nerve.  —  The  nerve  supplying  the  side  of  the  head  with  sensations, 
194. 

Fallopian  tubes. — •  In  mammals  the  part  of  the  oviduct  extending  from  the 
ovary  to  the  uterus. 

family. —  A  group  of  similar  genera,  372. 

fat. —  One  of  the  three  chief  food  substances;  a  hydrocarbon  made  up  of 
a  fatty  acid  and  glycerine,  9,  133. 

fatty  acid. —  One  of  the  materials  into  which  fat  may  be  decomposed,  10. 

fauna. —  The  total  animal  life  of  any  region. 

Felis,  372. 

females. — Individuals  producing  eggs,  251. 

female  pronucleus. —  The  matured  egg  nucleus  before  union  with  the 
sperm,  254. 

female  spores,  122. 

ferment. —  Chemical  substances  that  produce  fermentation;  enzymes,  306. 

fermentation,  79. 

fern,  life  history  of,  269. 

fertilization. —  The  union  of  the  egg  nuclei  and  the  sperm  nuclei,  122, 
249,  251,  257,  263.  In  botany  the  term  is  frequently  erroneously 
applied  to  the  transference  of  pollen  to  the  pistil,  277. 

fibers. —  The  individual  elements  of  muscles  and  nerves,  170. 

fibrillae. —  The  minute  filaments  of  which  a  muscle  fiber  is  composed. 

fibrin. —  A  proteid  obtained  from  clotted  blood. 

fibrovascular  bundles  (Lat.  fibra  =  fiber  -f-  E.  vascular). —  Bundles  of  long 
cells  of  various  shapes,  extending  lengthwise  and  strengthening  the 
stems  of  the  higher  plants,  104. 

fibula,  182. 

filament. —  The,  thread-like  stem  to  a  stamen,  119;  any  thread-like  organ. 

fission  (Lat.  findere  =  to  split). —  Division  into  two  equal  halves,  58,  63. 

flagella  (Lat.  flagellum  =  a  whip). —  Rather  long,  lashing  processes  of  pro- 
toplasm, one  to  six  to  each  cell,  73,  218. 

flexors. — •  Muscles  that  bend  the  joints,  211. 

flexure. —  A  bending. 

Flora. —  The  total  vegetation  of  any  territory. 

flowers,  118. 

foam  theory  of  protoplasm,  31. 


GLOSSARY-INDEX  401 

foetus. —  The  embryo  while  within  the  uterus  of  the  mother,  291. 

follicle. —  The  pocket  in  which  a  hair  is  produced. 

foods  of  plants,  126. 

food  vacuoles. —  Clear  spaces  in  Protozoa  representing  the  remains  of 

digested  food, 
foramen  magnum    (Lat.  foramen  =  opening  +  magnum  =  great). —  The 

opening  into  the  skull  through  which  the  spinal  cord  enters,  180. 
forebrain. —  The  front  part  of  the  brain,  consisting  of  cerebrum  and  thala- 

mencephalon,  193. 

fore-gut. —  The  front  part  of  the  alimentary  canal;  the  stomodceum. 
fossils. — •  The  remains  of  animals  or  plants  found  in  the  rocks,  383. 
fourth  ventricle,  193. 
free-living,  227. 

frond,—  The  leaf  of  a  fern,  269. 
frontal  bone,  180. 
fundamental  cells. —  The  cells  which  make  up  the  bulk  of  the  stem  of 

young  plants;  they  are  roughly  spherical  in  shape  and  never  elongated, 

104. 

Fungi,  significance  in  nature,  134,  234. 
fusion  nucleus. —  The  nucleus  formed  from  the  fusion  of  two  nuclei  into 

one,  as  in  fertilization  or  conjugation,  65,  241,  257. 
gall  bladder. —  The  sac  which  temporarily  stores  the  bile,  187. 
gamete  (Gr.  gamete  =  husband  or  wife). —  One  of  the  uniting  cells  in  sex 

union;  usually  male  or  female,  but  sometimes  not  showing  any  sex 

differentiation,  262,  267. 
gametophyte    (Gr.    gamete  =  husband   or  wife  +  phyton  =  plant) . — •  The 

stage  in  the  life  cycle  of  a  plant  that  produces  sex  organs,  272,  275. 
ganglion. —  A  group  of  aggregated  neuron  bodies,  162. 
gastric  glands,  204. 
gastric  juice. — -The  digestive  secretion  produced  in  the  walls  of  the  stomach, 

204. 

gastrovascular  cavity. —  The  cavity  in  the  body  of  Ccelenterata,  141. 
gastrula  (Gr.  gaster  =  a  stomach). —  An  early  stage  in  the  embryology  of 

animals.     See  page  285. 

gemmae  —  Special  buds  formed  for  reproduction,  gemmules,  243. 
gemmation. —  The  same  as  budding,  239. 
gemmules. —  Special  buds  which  break  away  from  the  parent  and  become 

new  individuals;  same  as  gemmae, 
generation. —  The  whole  life  history  of  an  organism,  from  any  stage  to  the 

same  stage  again,  67. 

genital  (Lat.  genere  =  to  produce) .— Pertaining  to  reproduction, 
genus  (pi.  genera). —  A  group  of  similar  species,  371. 


402  BIOLOGY 

geotropism  (Gr.  ge  =  earth  -f  trope  =  a  turning). —  The  power  possessed 
by  many  plants  of  growing  toward  or  away  from  the  earth. 

germinal. —  Pertaining  to  reproduction. 

germ  layers. —  The  three  layers  formed  in  the  developing  embryo,  ^83. 

germ  plasm. —  The  substance  which  bears  the  hereditary  traits  and  is  con- 
tinuous from  generation  to  generation,  330. 

gills. —  Thin,  expanded  organs,  bathed  in  water  for  respiratory  purposes, 
288. 

gill  slits. —  See  branchial  openings. 

girdling,  111,  129. 

gizzard. —  A  muscular  chamber  of  the  digestive  tract  where  food  is  ground, 
158. 

glands. —  Groups  of  cells  which  produce  secretions,  167,  176,  204. 

glenoid  cavity,  182. 

glomerulus. —  See  Malpighian  body. 

glossopharyngeal  (Gr.  glossa  =  tongue  -f  pharynx). —  A  nerve  from  the 
brain  supplying  the  tongue  and  throat,  194. 

glottis. —  The  opening  of  the  trachea  or  larynx  into  the  mouth,  186. 

glucose. —  A  sugar  from  fruits,  or  artificially  made  from  starch,  containing 
maltose  and  dextrin,  9. 

gluten. —  A  proteid  from  cereals,  8. 

glycerine. —  One  of  the  decomposition  products  of  fat,  10. 

gonads. —  Glands  producing  eggs  or  sperms,  251. 

Gonium,  240. 

grafting. —  Inserting  a  part  of  one  animal  or  plant  into  another  in  such  a 
way  that  the  inserted  part  retains  its  life  and  grows. 

granular. —  Filled  with  granules  or  minute  solid  particles. 

granular  theory  of  protoplasm,  31. 

gregarious. —  Congregating. 

growth,  4. 

guard  cells,  116. 

gullet. —  The  oesophagus,  158. 

gustatory. —  Pertaining  to  taste. 

gyncecium. —  Same  as  pistil. 

haemal  (Gr.  haima  =  blood). —  Pertaining  to  the  blood. 

haemoglobin  (Gr.  haima  =  blood  +  Lat.  globus  =  globe). —  A  red  proteid 
which  colors  the  blood  red,  158,  192,  209. 

hair  follicle. —  The  tiny  pocket,  within  which  each  hair  grows. 

hairs,  35,  117. 

hallux. —  The  great  toe. 

hand,  365. 

hare,  34£. 


GLOSSARY-INDEX  403 

Haversian  canals. —  The  canals  in  bone  in  which  the  blood  vessels  run. 

heart,  187,  206,  309;  of  earthworm,  159. 

heliotropism    (Gr.    helios  =  the   sun  +  trope  =  a   turning). —  The   power 

possessed  by  plants  of  turning  toward  the  sun. 
helotism  (Gr.  helot  =  a  slave) . —  An  association  of  organisms  in  which  one 

enslaves  the  other,  228. 

hepatic  (Gr.  hepar  =  liver). —  Pertaining  to  the  liver, 
hepatic  vein. —  The  vein  from  the  liver,  190. 
herbivorous  (Lat.  herba  =  grass  +  vorare  =  to  eat). —  Feeding  upon  grass, 

herbs  or  other  plants, 
heredity. —  The  appearance  in  the  offspring  of  characters  of  the  parent, 

326;  nucleus  in,  48,  259;  Weismann's  theory  of,  329. 

hermaphrodites. —  Individuals  possessing  both  male  and  female  reproduc- 
tive glands,  251. 

heterocercal. —  Applied  to  a  tail-fin  with  one  lobe  longer  than  the  other, 
heterosporous  (Gr.  heteros  =  other  -f  spore). —  Producing  more  than  one 

kind  of  spore;  i.  e.,  macrospores  and  microspores,  274. 
hibernation  (Lat.  hibernare  =  to  winter). —  The  death-like  sleep  which  some 

animals  show  in  winter,  214. 

high  organisms. —  Organisms  with  complex  structure,  96. 
hind-brain. —  The  posterior  part  of  the  brain,  consisting  of  cerebellum  and 

medulla,  193. 
hind-gut. —  The  hind  part  of  the  intestine,  the  cloacal  chamber;  also  called 

the  proctodoeum. 
hinge  joint,  185. 
histology   (Gr.  histos  =  a  web  -f  logos  =  discourse). —  The  study  of  the 

microscopical  anatomy  of  organisms,  19;  of  earthworm,  166. 
holophyte    (Gr.   holos  =  whole  +  phyton  =  a  plant). —  Having  the  food 

habits  of  plants,  i.  e.,  capable  of  utilizing  sunlight  and  assimilating 

CO2,  221. 
holozoic  (Gr.  holos  =  whole  -f  zoon  =  animal). —  Having  the  food  habits 

of  animals,  i.  e.,  nourished  wholly  on  organic  foods,  221. 
homocercal. —  Applied  to  a  tail-fin  with  both  lobes  equal, 
homologous  (Gr.  homos  =  like  +  logos  =  ratio). —  Similar    in    structure, 

364. 
homosporous  (Gr.  homos  =  like  +  spore). —  Producing  only  one  kind  of 

spore,  274. 
HOOKER,  38. 
Horse,  foot  of,  365. 
host. A  name  applied  to  an  animal  or  plant  upon  which  another  is  living 

as  a  parasite,  227, 
humerus,  182. 


404  BIOLOGY 

humor. —  A  name  applied  to  the  transparent  liquids  in  the  eye,  197. 

HUXLEY.  40. 

hybrids. —  Organisms  resulting  from  the  crossing  of  different  species,  268. 

Hydatina,  57,  247. 

Hydra,  description  of,  140. 

hydrocarbons,  9. 

hydroids. —  Animals  closely  related  to  Hydra,  148,  277. 

hydrophyte    (Gr.    hydor  =  water  +  phyton  =  plant). —  Plants    living    in 

water  or  in  a  very  wet  habitat, 
hyoid. —  A  V-shaped  arch  of  bone  under  the  jaw  and  surrounding  the  larynx, 

181. 
hyomandibular. —  A  chain  of  bones  attaching  the  lower  jaw  to  the  skull 

hi  the  frog. 

hypha. —  One  of  the  filaments  of  a  mycelium, 
hypoblast. —  Applied  to  the  endodermal  layer  in  the  embryo, 
hypophysis  (Gr.  hypo  =  under  +  phuein  =  nature). —  See  pituitary  body, 

193. 

ileum. —  A  name  given  to  the  intestine  below  the  duodeum. 
ilium,  182. 

imago. —  The  adult  stage  of  an  insect  with  a  metamorphosis,  289. 
imbibition  (Lat.  imbibire  =  to  imbibe). —  The  action  of  absorbing  water, 

shown  by  many  organic  substances, 
immutability  of  species   (Lat.  in  =  not  +  mutabilis  =  changing). —  The 

theory  that  species  remain  constant,  349. 
imperfect  flowers. —  Flowers  in  which  either  stamens  or  pistils  are  lacking, 

120,  121. 
impregnation  (Lat.  impregnare  =  to  make  pregnant). —  See  fertilization, 

257. 
inbreeding. —  Breeding  from  a  male  and  female  of  the  same  parentage,  like 

brother  and  sister,  268. 
income,  of  an  animal,  219;  of  a  plant,  220. 
incubation  (Lat.  incubare  =  to  he  on). —  To  keep  warm. 
Indirect  development,  290. 
individual,  67. 

individual  variations,  337,  358. 
indusium  (Lat.  induere  =  to  put  on). —  A  covering  over  the  sporangia  in 

the  sorus  of  a  fern,  269. 
inferior  vena  cava. —  The  large  venous  trunk  bringing  the  blood  from  the 

lower  parts  of  the  body  to  the  heart:  same  as  posterior  vena  cava,  190. 
infundibulum. —  Any  funnel-shaped  or  dilated  organ,  193. 
infusion. —  A  preparation  made  by  steeping  a  substance  like  hay  in  warm 

water. 


GLOSSARY-INDEX  405 

inner  ear,  198. 

inorganic,  26. 

insertion. —  The  attachment  of  a  muscle  farthest  from  the  center  of  the 

body,  184. 

intercellular  (Lat.  inter  =  between  +  cellular). —  Lying  between  the  cells, 
intercellular  digestion,  145. 
internodes. —  Spaces  between  the  nodes, 
interstitial    cells    (Lat.    inter  =  between  +  sistere  =  to    set). —  Cells    in 

Hydra  lying  between  the  cnidoblasts  and  the  muscle  cells,  143. 
intestine. —  The  digestive  tract  from  stomach  to  cloacal  chamber,  186. 
intracellular  (Lat.  intra  =  within  +  cellular). —  Lying  within  the  cells, 
intracellular  digestion,  146. 
intussusception    (Lat.    intus  =  inside  -f-  suscipere  =  to    take    up). —  The 

process  of  growth  by  taking  material  inside  the  body  and  incorporating 

it  into  the  body  substance,  5. 
invagination  (Lat.  in  =  within  +  vagina  =  a  sheath). —  The  act  of  turning 

inward,  as  when  the  finger  of  a  glove  is  pushed  into  the  palm, 
invertebrata. —  A  name  given  to  all  animals  below  vertebrata. 
invertion. —  The  splitting  of  a  molecule  of  cane  sugar  into  two  molecules 

of  grape  sugar,  a  molecule  of  water  being  added  in  the  process,  9. 
iris. —  An  opaque  curtain,  containing  pigment,  covering  the  front  of  the 

eyeball,  197. 
irritability. —  The  power  of  reacting  under  the  influence  of  stimuli,  43,  57, 

63,  219. 
ischia,  182. 
isolation. —  The  separation  of  two  individuals  from  the  rest  of  the  species 

so  that  they  will  breed  together,  351. 
jellyfish. —  See  medusa. 
joints. —  Places  where  bones  or  other  hard  movable  parts  come  together, 

176,  184. 

karyokinesis  (Gr.  karyon  =  nucleus  +  kinesis  =  movement). —  The  proc- 
ess of  cell  division  accompanied  by  a  peculiar,  complicated  nuclear 

division;  mitosis,  85. 
karyoplasm  (Gr.  karyon  =  nucleus  +  plasma  =  substance). —  The  liquid 

protoplasm  inside  the  nucleus;  nucleoplasm,  32,  49. 
katabolism,  139. 

kidneys. —  Glands  in  vertebrates  secreting  urea,  199. 
kinetic  energy  (Gr.  kinetos  =  moving). —  Energy  in  motion;  active  energy, 

293. 

kingdoms.  —  The  two  divisions  of  organisms,  animals  and  plants,  373. 
lachrymal  (Lat.  lachryma  =  a  tear). —  Pertaining  to  tears, 
lacteals. —  Lymph  vessels,  carrying  absorbed  fat  from  the  intestine,  192. 


406  BIOLOGY 

lacunae. —  Spaces  among  the  tissues  in  which  lymph  collects,  192,  208. 

L..MARCK,  350. 

Lamarckian  factors.  —  Forces  in  evolution  first  suggested  by  Lamarck, 

351. 

lamella. —  A  thin  plate  or  layer, 
larva. — A  free-living  stage  in  the  development  of  an  animal,  unlike  the  adult, 

e.  g.,  a  tadpole,  286,  289. 
larval  history. —  The  stages  in  the  life  history  of  an  animal  after  hatching 

from  the  egg  and  before  adult  form  is  reached, 
larynx. —  The  enlargement  of  the  air  passages  containing  the  vocal  cords, 

191,  209. 

leaf,  structure  of,  114. 

legumen. —  A  proteid  derived  from  legumes,  8. 
legumes. —  Plants  belonging  to  the  Leguminosae  family,  like  beans,  peas, 

clover,  alfalfa,  vetches,  locusts,  etc. 
Leucanthemum,  345. 
leucocytes  (Gr.  leukos  =  white  -f-  cytos  =  cell). —  The  white  corpuscles  of 

the  blood,  192,  205. 
lichens. —  The  grayish  green  mosses  which  grow  on  rocks  or  trees,  etc.;  an 

association  of  a  fungus  and  a  green  plant,  229. 
life  cycle. —  See  generation;  of  nature,  234. 
life  force,  4,  323. 

ligaments. —  Bands  of  connective  tissue  connecting  bones,  184. 
lingual  (Lat.  lingua  =  tongue). —  Pertaining  to  the  tongue,  190. 
linin. —  The  delicate  fibers  extending  through  the  karyoplasm  and  forming 

a  network,  32. 

littoral. —  Pertaining  to  the  shore. 

liver. —  A  large  gland  opening  into  the  intestine  at  the  pylorus,  186. 
lophotrichic  (Gr.  lophos  =  a  crest  +  thrix  =  hair). —  With  a  tuft  of  fla- 

gella,  81. 

low  organisms. —  Organisms  with  a  simple  structure,  96. 
lumen. —  A  cavity  in  a  tube  or  sac. 
lungs,  191,  209. 

lymph. —  The  liquid  part  of  the  blood  after  it  has  passed  out  of  the  capilla- 
ries into  the  tissues,  176,  192,  208. 
lymph  glands. —  Glandular  swellings  on  the  lymph  vessels,  which  belong  to 

the  ductless  glands;  lymph  nodes,  192. 
lymph  hearts. —  Four  pulsating  sacs  in  the  frog,  that  force  lymph  into  the 

veins,  192,  208. 

lymph  spaces. —  Spaces  in  tissues  in  which  lymph  collects,  176. 
lymph  vessels. —  Tubes  carrying  lymph  from  the  lacunae  to  the  veins,  192, 

208. 


GLOSSARY-INDEX  407 

machine. —  Any  mechanism  designed  to  convert  one  kind  of  energy  into 

another,  297. 
macronucleus  (Gr.  macros  =  large). —  The  larger  of  the  two  nuclei  in  cells 

having  two,  62. 
macrospore   (Gr.  macros  —  large).— The  large  spores  ;n  certain  plants, 

which  develop  into  female  gametophytes,  122,  273. 
macula  lutea  (Lat.  macula  =  spot  +  luteus  =  yellow). —  A  small  spot  on 

the  retina,  with  most  acute  vision, 
malaria,  69,  232. 

males. —  Individuals  producing  sperms,  251,  257. 
male  pronucleus. —  The  sperm  after  entering  the  egg  and  before  it  unites 

with  the  egg  nucleus,  257. 
male  spores,  122. 
Malpighian  bodies. —  Minute  rounded  bodies  in  the  kidneys  filled  with  a 

knot  of  blood  vessels;  glomeruli. 
Mammalia  (Lat.  mamma  =  breast). —  Animals,  the  females  of  which  have 

milk  glands,  373. 

mammary  glands. —  Glands  secreting  milk,  373. 
mandible. —  The  jaw  bone,  180;  also  the  jaw-like  teeth  of   animals  like 

insects  and  Crustacea. 

mantle. —  A  fold  of  skin  more  or  less  enveloping  the  body  of  an  animal, 
marrow. —  The  soft  material  filling  the  cavities  of  bones, 
maturation  (Lat.  maturare  =  to  make  ripe). —  The  final  changes  by  which 

an  egg  becomes  prepared  for  fertilization,  253. 
maxilla. —  A  bone  forming  the  upper  jaw,  180;  also  mouthparts  of  insects 

or  Crustacea, 
mechanical  theory. —  The  theory  that  life  phenomena  are  manifestation? 

of  chemical  and  mechanical  forces  only,  41. 
medulla  oblongata. —  The  posterior  part  of  the  brain,  193. 
medullary  rays  (Lat.  medulla  =  marrow) . —  Bundles  of  cells  extending  from 

the  center  to  the  outer  parts  of  a  stem,  111. 
Medusae. —  The  sexual,  free-swimming  stage  of  certain  hydroids  and  other 

Ccelenterata;  jelly  fishes,  144,  278. 
megaspore. —  Same  as  macrospore. 

membrane  bones. —  Bones  formed  first  as  membranes,  181. 
membranella. —  A  band  of  fused  cilia  found  in  some  of  the  unicellular 

animals,  61. 

Mendelism. —  A  law  of  heredity  first  advanced  by  Mendel,  359. 
mental  functions,  317. 

mesenteron  (Gr.  mesos  =  middle  +  enteron  =  intestine) . — The  mid-gut,  284. 
mesentery   (Gr.  mesos  =  middle  +  enteron  =  intestine). —  A  fold  of  the 

peritoneum  which  slings  the  intestine  in  position,  187. 


408  BIOLOGY 

mesoblast. —  The  mesoderm  of  the  developing  embryo. 

mesoderm  (Gr.  mesos  =  middle  +  derma  =  skin). —  The  middle  layer  in 

a  developing  embryo,  283. 

mesogloea  (Gr.  mesos  =  middle  +  gloios  =  glue). —  The  middle,  non-cellu- 
lar layer  of  Hydra  and  allied  animals,  143. 
mesophyll  cells   (Gr.   mesos  =  middle  +  phyllon  =  leaf). —  The  irregular, 

loosely  packed,  chlorophyll  cells  in  the  middle  of  a  leaf,  117. 
xnesophytes  (Gr.  mesos  =  middle  -f  phyton  =  plant). —  Plants  living  in  a 

moderately  moist  habitat. 
metabolism  (Gr.  meta  =  beyond  +  ballein  =  to  throw) . —  A  name  given 

to  the  series  of  chemical  changes  going  on  in  organisms,  138,  210. 
metacarpals,  182. 
metameres  (Gr.  meta  =  beyond  -j-  meros  =  a  part). —  Segments  of  animals 

like  the  earthworm,  155. 
metamorphosis  (Gr.  meta  =  beyond  +  morphe  =  form). —  A  life  history  in 

which  an  organism  passes  through  several  unlike  stages,  more  or  less 

independent,  72,  289;  of  frog,  280,  286. 
metaphase. —  The  second  step  in  karyokinesis,  87. 
Metaphyta    (Gr.    meta  =  beyond  +  phyton  =  plant). —  Plants    made    of 

many  cells,  223. 
metastasis  (Gr.  meta  =  beyond  +  histanai  =  to  place.) —  The  process  of 

using  foods,  132,  135,  300. 
metatarsals,  182. 
Metazoa  (Gr.  meta  =  beyond  +  zoon  =  animal). —  Animals  made  of  many 

cells,  223. 

mice,  breeding  of,  361. 
micronucleus  (Gr.  mikros  =  small  +  nucleus). —  The  small  nucleus  in  a 

cell  containing  two  nuclei,  62. 

microorganism. —  Any  organism  of  microscopic  size, 
microsomata   (Gr.  mikros  =  small  +  soma  =  body). —  Extremely  minute 

bodies  in  the  protoplasm  which  frequently  show  motion,  32. 
microspores  (Gr.  mikros  =  small  +  spore). —  The  small  spores  in  a  plant, 

which  develop  into  male  gametophytes,  122,  273. 
mid-brain. —  The  middle  part  of  the  brain,  consisting  of  the  optic  lobes 

(called  corpora  quadrigemina  in  man),  193. 
migration.—  The  act  of  changing  one's  dwelling  place  from  one  locality  to 

another,  379. 
mimicry. —  Resemblances  which  some  organisms  show  to  other  objects,  for 

protective  purposes. 

mitosis  (Gr.  miios  =  a  thread). —  See  karyokinesis. 
mitral  valve. —  The  valve  between  the  left  auricle  and  vent-ride, 
molds,  96,  99,  235. 


GLOSSARY-INDEX  409 

molecule. —  The  smallest  particle  of  a  chemical  compound  which  can  exist 
without  the  compound  being  chemically  destroyed. 

Monocystis,  241. 

monoecious  (Gr.  monos  =  one  -j-  oikos  =  house). —  With  both  sexes  in  the 
same  individual;  applied  to  plants,  251. 

monogamous  (Gr.  monos  =  one  +  gamos  =  marriage). —  The  sexual  asso- 
ciation of  one  male  with  one  female. 

monotrichic  (Gr.  monos  =  one  +  thrix  =  hah1) . —  With  a  single  flagellum, 
81. 

morphology  (Gr.  morphe  =  form  +  logos  =  discourse) . —  The  study  of  the 
structure  of  organisms  in  all  relations,  19. 

morula  (Lat.  morum  =  a  mulberry). —  The  stage  in  the  egg  development 
after  the  egg  has  become  a  sphere  of  cells. 

motion,  in  plants,  136,  218;  in  the  earthworm,  167;  in  the  frog,  211. 

motor  cells. —  The  neurons  which  send  impulses  over  their  axons  to  the 
muscles  to  produce  motion,  172,  213. 

motor  ocularis. —  The  third  cerebral  nerve  supplying  the  eye  muscles,  194. 

Mucor,  97,  247. 

mucous  membrane. —  The  lining  of  the  alimentary  canal,  187. 

mucous. —  Applied  to  glands  secreting  mucus. 

mucus. —  A  thick,  viscid  secretion  from  the  mucous  membrane. 

multicellular  organisms  (Lat.  multus  =  many  +  cellular). —  Organisms 
made  of  many  cells  which  show  a  differentiation  among  themselves, 
90,  95. 

muscles,  219. 

mutations  (Lat.  mutare  =  to  change). —  Sudden  departures  from  the  race 
character  which  have  a  tendency  to  remain  fixed,  358. 

mutation  theory  (Lat.  mutare  =  to  change).—  The  theory  of  evolution  that 
assumes  that  progress  has  taken  place  by  mutations  rather  than  by 
individual  diversities,  357. 

mutualism. —  An  associating  of  organisms  for  mutual  benefit,  228. 

mycelium  (Gr.  mykes  =  fungus  +  helos  =  a  nail). —  The  thread-like  fila- 
ments of  which  fungi  are  composed,  96. 

myosin  (Gr.  raws  =  muscle). —  A  proteid  in  lean  meat,  8. 

nares. — •  See  nostrils. 

nasal  bones,  180. 

natural  selection.  —  The  law  by  which  the  best  fitted  organisms  survive. 
353. 

NEEDHAM,  13. 

nematocysts  (Gr.  nema  =  a  thread  +  cystis  =  sac). —  Special  cells  in  Cce- 
lenterata  which  have  a  coiled  poison  thread  capable  of  extrusion;  net- 
tling cells,  143. 


410  BIOLOGY 

nephridia  (Gr.  nephros  =  kidney). —  The  organs  of  the  earthworm  which 
excrete  nitrogenous  waste,  161. 

nerve  fibers. —  The  separate  fibers  of  which  a  nerve  is  composed,  170. 

nerve  impulse,  314. 

nerves,  163,  194. 

nervous  system  of  earthworm,  162;  of  frog,  192;  of  Hydra,  146. 

netted-veined  leaves. —  Those  with  veins  branching  into  a  network,  114. 

nettle-hairs,  117. 

nettling  cells. —  See  nematocysts. 

neural  arch  (Gr.  neuron  =  a  nerve). —  The  arch  of  bones  on  top  of  the  ver- 
tebrae, inclosing  the  neural  foramen,  177. 

neuroglia  (Gr.  neuron  =  a  nerve  +  gloios  =  glue). —  The  connective  frame- 
work of  the  nervous  system. 

neurons  (Gr.  neuron  =  a  nerve). —  The  nerve  cells  which  are  the  units  of 
the  nervous  system,  169,  195. 

nictitating  membrane  (Lat.  nictare  =  to  wink). —  A  semitransparent,  inner 
eyelid  in  the  frog  and  some  other  animals,  175. 

nidamental  glands  (Lat.  nidus  =  a  nest). —  Glands  connected  with  th<> 
oviduct,  that  secrete  the  covering  of  eggs,  201. 

Nitetta,  29. 

nitrification. —  The  production  of  nitrates  in  the  soil  from  simpler  nitrogen 
compounds. 

nodes. —  The  places  on  a  stem  where  branches  arise. 

nostrils. —  Openings  into  the  nasal  cavities;  nares,  175,  186. 

notochord  (Gr.  notos  =  the  back  -f  chorde  =  a  string). —  A  rod  in  the  back 
of  vertebrate  embryos  that  develops  into  the  spinal  column,  286. 

nucleolus. —  A  small  body  in  the  nucleus  of  a  cell,  with  unknown  functions, 
32. 

nucleoplasm. —  See  karyoplasm. 

nucleus. —  The  vital  center  of  a  cell,  containing  chromatin  and  controlling 
constructive  metabolism,  29,  32,  45. 

occipital  bones. —  The  skull  bones  which  surround  the  foramen  magnum, 
180. 

occipital  condyles. —  The  rounded  protuberances  by  which  the  skull  articu- 
lates with  the  first  vertebra,  181. 

oesophagus. —  The  tube  from  the  throat  to  the  stomach  or  crop,  or  from 
the  mouth  into  the  body,  60,  158. 

olfactory  lobes. — Two  small  lobes  of  the  brain  in  front  of  the  cerebrum,  193. 

olfactory  nerve. —  The  first  of  the  cerebral  nerves,  supplying  the  olfactory 
sacs,  194,  196,  198. 

olfactory  sacs. —  Minute  sacs  in  the  nasal  cavities;  the  seat  of  the  sense  of 
196,  198. 


GLOSSARY-INDEX  411 

omosternum  (Gr.  omos  =  shoulder  +  sternon  =  the  chest). —  A  bit  of 
cartilage  forming  the  front  of  the  sternum  in  the  frog,  182. 

ontogeny  (Gr.  ont  =  being  +  -geneia  =  a  producing). —  Development  from 
the  egg,  19,  290. 

oocyte  (Gr.  oon  =  egg  +  cytos). —  An  egg  before  maturation,  252. 

oogenesis  (Gr.  oon  =  an  egg  +  genesis  =  creation). —  The  development  of 
the  egg  in  the  ovary,  252. 

oogonium  (Gr.  oon  =  an  egg  +  gonos  =  offspring). —  A  sac  in  some  plants 
within  which  are  produced  one  or  two  eggs. 

operculum. —  A  lid-like  cover. 

optic  lobes. —  The  section  of  the  brain  in  front  of  the  cerebellum;  the  mid- 
brain,  193. 

optic  nerve. —  The  second  cerebral  nerve,  supplying  the  eye,  194,  196. 

optimum  temperature,  132. 

oral. —  Pertaining  to  the  mouth,  60. 

order. —  A  group  of  similar  families,  372. 

organ. —  Any  part  of  an  animal  or  plant  adapted  for  a  specific  function; 
usually  made  of  a  combination  of  several  different  tissues,  26,  95. 

organic  evolution,  349. 

organic  substances.—  Substances  originally  derived  from  organisms,  26; 
in  chemistry,  any  compounds  of  carbon. 

organism. —  A  living  being  provided  with  organs;  hence  any  living  being, 
26. 

organisms  as  machines,  298. 

origin. —  The  attachment  of  a  muscle  nearest  the  center  of  the  body,  184. 

Oscittaria,  136,  219. 

osmosis. —  The  force  that  causes  some  substances  to  diffuse  through  mem- 
branes which  are  moistened  on  both  sides;  dialysis,  127,  307. 

ossification. —  Turning  to  bone. 

osteoblast  (Gr.  osteon  =  a  bone  +  blastos  =  a  sprout). —  A  bone-forming 
cell. 

otic. —  Pertaining  to  the  ear,  180. 

otocyst  (Gr.  ous  =  the  ear  +  cystis  =  a  sac). —  A  sac  which  in  many 
invertebrates  is  supposed  to  have  hearing  functions. 

otoliths  (Gr.  ous  =  the  ear  -f  lithos  =  a  stone). —  Calcareous  bodies  found 
in  the  otocysts  in  some  animals. 

outgo,  of  an  animal,  220;  of  a  plant,  220. 

ova. —  The  female  reproductive  cells,  249,  267. 

ovary. —  In  animals  the  glands  producing  eggs,  151, 163,  200,  249;  in  flowers 
the  lower  part  of  the  pistil  containing  the  ovules  and  seeds,  120. 

overproduction,  353. 

oviducts. —  Ducts  for  carrying  eggs  to  the  exterior,  163,  200,  249. 


412  BIOLOGY 

oviparous  animals.— Animals  that  lay  eggs,  290. 

ovules. —  The  small  bodies  in  the  pistil  that  contain  the  macrospores  and 
grow  into  the  seed,  121. 

oxidation. —  Union  with  oxygen,  as  in  ordinary  combustion,  55,  138. 

palatine  bones,  180. 

paleontology  (Gr.  palaios  =  ancient  +  out  =  being  +  logos'). —  The  study 
of  the  distribution  of  organisms  in  the  past  ages  by  means  of  fossils, 
383. 

palisade  cells. —  A  layer  of  regular,  chlorophyll  cells,  just  beneath  the  upper 
epidermis  in  most  leaves,  117. 

pallium.—  See  mantle. 

pancreas. —  A  digestive  gland  opening  into  the  intestine  just  below  the 
stomach,  187,  204. 

pancreatic  fluid  or  juice. —  The  secretion  of  the  pancreas,  204,  306. 

Pandorina,  73,  90,  263. 

papilla. —  A  small  finger-like  projection. 

parallel-veined  leaves. —  Those  with  veins  running  from  base  to  tip,  or 
from  midrib  to  margin,  in  a  roughly  parallel  course,  114. 

Paramecium,  description  of,  59. 

parasite. —  An  organism  that  lives  upon  and  feeds  upon  a  living  host,  227. 

parasitism,  effect  of,  231. 

parasphenoid  bones,  180. 

parenchyma  (Gr.  para  =  beside  +  enchein  =  to  pour  in). —  Short,  square- 
ended  cells  in  plants,  106. 

parietal  bones,  180. 

parotids. — •  Salivary  glands  in  front  of  the  ear  in  some  animals. 

parthenogenesis  (Gr.  parthenos  =  virgin  +  genesis  =  a  creation). —  Re- 
production by  eggs  which  do  not  require  fertilization,  246,  262,  265. 

PASTEUR,  14. 

PASTEUR'S  solution,  83. 

patheticus. —  The  fourth  cerebral  nerve,  supplying  the  eye  muscles,  194. 

pathogenic  (Gr.  pathos  =  disease  +  -genie). —  Disease  producing,  82. 

pectoral. —  Pertaining  to  the  chest. 

pedal  (Gr.  pous  =  a  foot). —  Pertaining  to  the  feet. 

peduncle. —  The  stalk  supporting  a  flower,  118. 

pelagic  (Lat.  pelagus  =  the  sea). —  Pertaining  to  the  open  ocean. 

pelvis. —  The  girdle  of  bones  attaching  the  legs  to  the  spinal  column,  182. 

Penicillium,  96. 

penis. —  The  male  copulatory  organ,  291. 

pepsin. —  A  ferment  in  the  gastric  juice. 

peptone. —  A  soluble  form  of  proteid. 

peptonize. —  To  convert  ordinary  proteids  into  peptones. 


GLOSSARY-INDEX  413 

Peranema,  75,  217. 

perennials  (Lat.  per  =  through  +  annus  =  year).— Plants  living  year  after 
year. 

perfect  flowers. —  Those  with  both  stamens  and  pistils,  121. 

perianth  (Gr.  peri  =  around  +  anthos  =  a  flower). —  A  name  given  to  the 
calyx  and  corolla  combined,  119. 

pericardium  (Gr.  peri  =  around  +  cardia  =  the  heart). —  A  sac  surround- 
ing the  heart,  187. 

perichondrium  (Gr.  peri  =  around  +  chondros  =  cartilage). —  Fibrous  ma- 
terial surrounding  cartilage. 

peripheral  system,  163. 

periosteum  (Gr.  peri  =  around  +  osteon  =  bone). —  Fibrous  material  sur- 
rounding bone. 

peristalsis  (Gr.  peri  =  around  +  stellein  =  to  place). —  The  writhing  mo- 
tions of  the  intestine,  205. 

peritoneum  (Gr.  peri  =  around  -f  teinein  =  to  stretch). —  The  membrane 
lining  the  abdominal  cavity,  167,  171,  187. 

peritrichic  (Gr.  peri  =  around  +  thrix  =  hah-). —  With  flagella  distributed 
over  the  body,  81. 

perivisceral  fluid  (Gr.  peri  =  around  +  Lat.  viscera). —  See  ccelomic  fluid. 

pes. —  The  foot. 

petals. —  Leaves  which  form  the  corolla,  119. 

petiole. —  The  stem  of  a  leaf,  114. 

phagocytes  (Gr.  phagein  =  to  eat  +  cytos  =  a  sac). —  Leucocytes  with  the 
power  of  absorbing  solid  objects,  205. 

phalanges. — -The  bones  of  the  fingers  and  toes,  182. 

phanerogams  (Gr.  phaneros  =  visible  +  gamos  =  marriage). —  Plants  which 
produce  flowers,  103. 

pharynx. —  The  throat  cavity,  158. 

phloem  (Gr.  phloios  =  inner  bark). —  The  bark,  105. 

photosynthesis  (Gr.  phos  =  light  +  synthesis  —  composition). —  The  func- 
tion of  starch  making,  possessed  by  green  plants  only,  129,  135,  218. 

phototropism  (Gr.  phos  =  light  +  trope  =  a  turning). —  Reaction  to  light, 
58. 

phyla. —  The  largest  subdivisions  of  animals  and  plants,  373. 

phytogeny  (Gr.  phylon  =  tribe  +  -geneia  =  a  producing).  — The  past  history 
of  organisms,  290. 

physiology  (Gr.  physis  =  nature  +  -logia). —  The  study  of  the  functions 
of  the  different  animals  and  plants,  19. 

pia  mater  (Lat.  piits  =  delicate  +  mater  =  mother). —  A  delicate  mem- 
brane surrounding  the  brain  and  cord,  inside  the  dura  mater,  194. 

pigeons,  338, 


414  BIOLOGY 

pigment  cells  (Lat.  pingere  =  to  paint). —  Cells  which  contain  coloring 

matter,  176. 
pineal  gland. —  A  small  body  lying  on  top  of  the  brain;  also  called  the  pineal 

eye;  same  as  epiphysis,  193. 

pistil. —  The  central  row  of  leaves  (carpels)  of  a  flower,  bearing  female  re- 
productive organs;  also  called  the  gyncecium,  120. 

pith. —  The  central  mass  of  cells  in  a  stem,  made  of  fundamental  cells,  104. 
pituitary  body. —  A  small  body  on  the  under  side  of  the  brain;  the  hypophy- 
sis, 193. 

placenta. —  The  membrane  by  which  the  embryo  is  attached  to  the  uterus 
in  mammals,  291;  in  plants,  the  line  of  attachment  of  seeds  in  the 
ovary, 
plankton  (Gr.  plankton  =  wandering). —  The  living  organisms  which  float 

in  water,  largely  microscopic. 

plantigrade. —  Walking  on  the  palms  of  the  hands  or  the  soles  of  the  feet, 
plasma. —  The  liquid  portion  of  circulating  blood,  191,  205. 
plasmodium  (Gr.  plasma  =  substance). —  A  jelly-like  mass. 
Plasmodium  malarice,  69,  239. 
plastids. —  Miscellaneous  bodies  within  a  cell,  37. 
platelets. —  Minute  bodies  in  the  blood  of  vertebrates,  192. 
pleura  (Gr.  pleura  =  a  rib). —  Membranes  surrounding  the  lungs.  . 

Pleurococcus,  77,  239. 

plexus  (Lat.  pleclare  =  to  weave). —  A  network  of  nerves,  194. 
pneumogastric  (Gr.  pneumon  =  lung  +  gaster  =  stomach). —  A  large,  cere- 
bral nerve  extending  down  the  neck  and  supplying  the  heart,  lungs,  and 
stomach,  194. 
Podocoryne,  277. 
poisons. —  Substances  which,  taken  into  the  body,  produce  injurious  effects, 

43. 

polar  cells. —  Small  cells  extruded  from  the  egg  during  its  maturation,  254. 
pollen. —  The  male  spores  produced  by  a  flower,  119. 
pollen  tube. —  An  outgrowth  from  a  pollen  grain  which  pushes  through  the 

style  of  a  flower  to  fertilize  the  egg  in  the  ovary,  122,  275. 
poll  ex. —  The  thumb. 

pollination. —  The  transfer  of  the  pollen  to  the  stigma,  277. 
polygamous  (Gr.  polus  =  many  +  gamos  =  marriage). —  The  sexual  asso- 
ciation of  one  male  with  several  females, 
polymorphism  (Gr.  polus  =  many  +  morphe  =  form). —  The  property  of 

having  two  or  more  forms  of  the  same  animal,  149. 
portal  circulation. —  The  circulation  of  blood  from  the  intestine  through  the 

liver;  it  has  two  capillary  systems,  190. 
portal  vein. —  The  vein  carrying  blood  from  the  intestine  to  the  liver,  190. 


GLOSSARY-INDEX  415 

posterior  end,  155. 

posterior  root. —  The  branch  of  the  spinal  nerve  entering  on  its  posterior 

side  and  carrying  impulses  toward  the  brain,  195. 
posterior  vena  cava.—  Same  as  inferior  vena  cava,  190. 
potential  energy,  293. 
precoracoid,  182. 

predatory. —  Living  by  preying  upon  other  animals, 
premaxillary  bones,  180. 
proboscis. —  An  elongated  portion  of  the  head  of  an  animal,  with  special 

functions. 

process. — •  Any  small  projection. 
procoelous    (Gr.   pro  =  before  -f  coilos  =  hollow). —  Applied   to   vertebrae 

which  are  concave  in  front  only, 
proctodaeum,  284. 

pronation. —  The  position  of  the  fore  arm  with  the  palm  downward, 
pronuclei. — •  The  two  nuclei,  male  and  female,  which  are  in  the  matured 

egg,  ready  to  unite  with  each  other,  254,  257. 
prophase.—  The  preliminary  stage  in  karyokinesis,  85. 
prostomium   (Gr.  pro  =  before  +  stoma  =  mouth). —  The  sensitive  lobe 

projecting  over  the  mouth  in  the  earthworm,  156. 
protective  resemblances. —  Resemblances  to  objects,   either  animate  or 

inanimate,  for  the  purpose  of  protection;  mimicry. 
proteids. —  Highly  complex  compounds  of  carbon,  oxygen,  hydrogen,  and 

nitrogen  and  some  other  elements;  the  basis  of  living  tissues  and  a 

necessary  part  of  animal  foods,  7,  133. 
prothallium  (Gr.  pro  =  before  +  thallos  =  a  branch). —  The  small,  sexual 

stage  of  the  life  history  of  a  fern,  271. 
protomitomic  theory,  50. 
Protophyta  (Gr.  protos  =  first  +  phyton  =  plant). —  The  unicellular  plants, 

222. 

protoplasm  (Gr.  protos  =  first  +  plasma  =  substance) . —  The  living  sub- 
stance of  organisms,  29,  30,  40,  48. 
Protozoa  (Gr.  protos  =  first  +  zoon  =  animal). —  The  unicellular  animals, 

92,  222. 

proximal. —  Nearest  to  the  body. 

pseudonavicellae  (Gr.  pseudes  =  false  +  Lat.  navicella  =  a  boat).— Spin- 
dle-shaped spores  formed  by  some  Sporozoa  as  the  result  of  the  union 

of  cells. 
pseudopodia   (Gr.  pseudes  =  false  -f  pous  =  foot).— Temporary  lobes  of 

protoplasm  used  in  locomotion,  52. 

psychology  (Gr.  psyche  =  the  soul  +  -logia). —  The  study  of  mind,  20. 
pterygoid  bones,  180. 


416  BIOLOGY 

ptyalin. —  The  enzyme  in  saliva  which  converts  starch  into  sugar. 

pubis,  182. 

puffballs,  245. 

pulmonary  arteries  (Gr.  pleumon  =  a  lung). —  Bloodvessels  carrying  blood 

to  the  lungs,  191. 

pulmonary  circulation. —  The  circulation  through  the  lungs,  191. 
pulmonary  veins. —  The  blood  vessels  carrying  blood  from  the  lungs  to  the 

heart,  191. 

pupa. — •  A  stationary,  inactive  stage  between  a  larva  and  an  adult,  289. 
pupil. —  An  opening  in  the  center  of  the  iris  allowing  light  to  enter  the  eye, 

197. 
putrefaction. —  Decomposition  of  organic  products,  taking  place  without 

the  presence  of  much  oxygen,  81. 

pylorus. —  The  opening  of  the  stomach  into  the  intestine,  186. 
quadrate  bones,  180. 
quadrato-jugal  bones,  181. 
rabbit,  skeleton  of,  364. 
race  variations. —  Variations  by  which  the  race  is  gradually  or  suddenly 

modified,  338. 

racemose. —  Arranged  somewhat  like  a  cluster  of  grapes, 
radiant  heat. —  Heat  which  is  given  off  from  a  hot  body  into  space,  297. 
radius,  182. 
ramus. —  A  branch. 

reaction. — A  response  to  an  external  stimulus,  43. 
recapitulation  theory. —  See  repetition. 
receptacle. —  In  botany,  the  end  of  the  flower  peduncle  on  which  the  floral 

leaves  are  borne,  118. 
recessive  characters  (Lat.  recessus  =  receding). —  Characters  which  fail  to 

appear  in  a  first  generation,  but    may  appear  in  later  generations, 

360. 

rectum. —  The  enlarged,  posterior  end  of  the  intestine,  186. 
REDI,  12. 
reflex  action. —  An  action  produced  by  a  stimulus  passing  to  the  central 

nervous  system  and  there  giving  rise  to  stimuli  which  pass  outward  to 

the  muscles,  but  without  volition,  212. 

regeneration. —  The  redevelopment  of  parts  that  have  been  lost,  150. 
reintegrate. —  To  recombine  compounds  that  have  been  disintegrated, 
renal. —  Pertaining  to  the  kidneys, 
renal  portal  vein. —  A  vein  from  the  legs  of  the  frog  that  breaks  up  into 

capillaries  in  the  kidney,  191. 
repetition,  law  of. —  The  law  that  the  development  of  animals  repeats  their 

past  history,  290. 


GLOSSARY-INDEX  417 

reproduction,  5,  45,  140,  238,  318,  rate  of. 

reproductive  cells,  267. 

reproductive  organs,  163,  199. 

reproductive  system  of  Amoeba,  58;  of  bacteria,  81;  of  earthworm,  163; 

of  Eudorina,  264;  of  frog,  214;  of  Hydra,  146,  151;  of  malarial  Plasmo- 

dium,  71;  of  Monocystis,  241;  of  Pandorina,  74;  of  Paramecium,  63;  of 

Penidllium,  98;  of  Ulothrix,  93;  of  yeast,  78. 

respiration. —  The  exchange  of  gases  between  organisms  and  their  environ- 
ment, 56,  138,  160,  225;  explained,  209,  312. 
reticular  theory  of  protoplasm,  31. 
reticulum. —  A  network,  32. 
retina. —  The  sensitive  part  of  the  eye,  196,  197. 
rhizoids. — •  Delicate  hairs  attaching  some  plants,  like  mosses,  to  the  soil, 

270. 

Ritinus  communis,  103. 
rigor  mortis   (Lat.   rigor  =  stiff  +  mors  = 'death). —  The  stiffening  that 

occurs  after  death, 
rivalries  of  organisms,  382. 
root  cap. —  A  protective  covering  of  hard  cells  over  the  tips  of  growing 

roots,  113. 
root  hairs. —  Delicate,  single-celled  absorption  hairs,  on  the  tips  of  roots 

of  plants,  113. 
root  pressure. —  The  pressure  of  sap  in  roots  that  forces  sap  up  the  stem, 

127. 

root  structure,  112. 

rudimentary  organs. —  Organs  only  imperfectly  developed, 
rusts,  232. 
Saccharomyces,  78. 
saccule,  198. 

sacrum. —  The  fused  vertebrae  between  the  hip  bones, 
salivary  glands. —  Glands  secreting  saliva,  204. 
saprophytes  (Gr.  sapros  =  rotten  +  phyton  =  a  plant). —  Plants  which  live 

upon  the  dead  bodies  of  other  organisms,  227. 

sarcode. —  A  name  first  given  to  the  living  contents  of  animal  cells,  40. 
scapula,  182. 

SCHLEIDEN,  38. 

SCHULTZE,  40. 
SCHWANX,  38. 
sciatic  plexus. —  The  network  formed  by  the  several  spinal  nerves  which 

combine  to  form  the  sciatic  nerve,  194. 
sclerenchyma  (Gr.  sderos  =  hard  +enchyma  =  infusion). — Plant  cells  wiih 

thick,  hard  walls. 


418  BIOLOGY 

sclerotic  coat  (Gr.  sderos  =  hard). —  The  outer  covering  of  the  eyeball,  196: 

sea  nettles. —  See  jelly  fishes. 

sebaceous  glands  (Lat.  sebum  =  fat). —  Oil  glands  in  the  skin. 

secreting  cells. —  Cells  which  extract  material  from  the  blood  and  secrete 

special  substances,  145,  161. 
secretions. —  Materials  eliminated  by  the  glands  and  used  by  the  body 

for  some  special  purpose,  e.  g.,  gastric  juice, 
seed. —  A  young  plant  surrounded  by  a  shell  and  lying  dormant;  developed 

in  higher  plants  only,  for  the  purpose  of  distribution,  122. 
seedling. —  The  young  plant  in  a  seed,  or  just  sprouting  from  a  seed,  123. 
segmentation. —  A  term  describing  the  division  of  the  earthworm  into 

segments,  155;  the  division  of  the  egg  into  many  cells  in  development 

(deavage),  280. 

segment. —  The  name  applied  to  the  rings  of  which  a  body  like  the  earth- 
worm is  composed;  melameres,  155. 
segregation   (Lat.  segregare  =  to  separate). —  The  grouping  together  of 

individuals  which  show  resemblances, 
semicircular  canals. —  Canals  in  the  inner  ear,  associated  with  the  sense 

of  equilibrium,  198. 
semilunar  valves. —  Valves  at  the  beginning  of  the  pulmonary  arteries  and 

the  aorta,  310. 

seminal  receptacles. —  Sacs  of  the  earthworm  for  holding  the  sperms  re- 
ceived at  copulation,  165. 
seminal  vesicles. —  Sacs  in  the  earthworm  for  holding  sperms  before  they 

are  ejected  during  copulation,  164,  200. 
sensation. —  A  conscious  feeling,  produced  in  the  brain  as  the  result  of 

impulses  reaching  it  from  the  various  sense  organs,  212,  316. 
sensations  in  plants,  137. 
sense  organs. —  Organs  at  the  outer  ends  of  the  nerves  which  are  excited 

by  external  stimulation,  167,  172,  195,  212. 
sensitiveness. —  Same  as  irritability,  219. 
sensitive  plants. —  Plants  which  respond  quickly  to  touch  by  closing  their 

leaves,  137. 

sepals. —  The  leaves  which  form  the  calyx,  119. 

septa. —  Partitions  separating  chambers,  especially  in  the  earthworm,  157. 
serous. —  Applied  to  glands  secreting  a  thin,  watery  liquid, 
serous  membranes. —  Membranes  lining  the  body  cavity  and  thorax, 
serum. —  The  liquid  part  of  the  blood  after  the  clot  has  separated, 
setae. —  Minute  bristles  serving  to  aid  the  earthworm  in  locomotion,  167. 
sexual  reproduction. —  Reproduction  by  union  of  eggs  and  sperms,  71. 

238,  240,  262;  distribution  of,  266.     In  earthworm,  163;  in  frog,  214; 

in  Hydra,  151;  in  plants;  origin  of,  263;  purpose  of,  335. 


GLOSSARY-INDEX  419 

sexual  stage. —  The  stage  in  a  metamorphosis  in  which  sexual  organs  are 
produced. 

shell,  of  an  egg,  250. 

shoulder  girdle,  181. 

sieve  cells. — •  Large  vessels  in  plants,  with  perforated  partitions  separating 
them  from  each  other,  106. 

sinus. —  Any  irregular  space  or  dilated  blood  vessel. 

skeleton,  139,  176. 

skin,  35,  176. 

skull,  180. 

sleep  of  plants,  137. 

smell. —  See  olfactory  organs. 

sociology  (Lat.  sotius  =  a  companion  +  Gr.  -logia). —  The  study  of  the 
relations  of  organisms  in  forming  societies,  20. 

somaplasm  (Gr.  soma  =  body  +  plasma  =  substance) . —  The  bit  of  the 
germ  substance  in  the  egg  that  is  set  aside  in  the  developing  egg  to  give 
rise  to  the  new  individual,  332. 

somatic  (Gr.  soma  =  body). —  Pertaining  to  the  body. 

sorus  (plural,  son). — A  cluster  of  sporangia  in  the  leaf  of  a  fern,  269. 

SPALANZANI,  13. 

special  creation  theory,  350. 

specialization. —  Adaptation  to  some  special  function. 

species. —  The  name  given  to  a  group  of  organisms  essentially  alike,  370. 

sperms. —  The  male  cells  in  sexual  reproduction,  250,  255,  267. 

spermaphytes. —  Seed-bearing  plants,  phanerogams,  376. 

spermaries. —  The  glands  that  produce  the  sperms,  164,  199,  250. 

spermatocyte  (Gr.  sperma  =  seed  +  cytos  =  cell). —  A  cell  in  the  spermary 
that  is  to  break  up  to  form  sperms,  254. 

spermatogenesis  (Gr.  sperma  =  seed  +  genesis  —  creation) .  —  The  devel- 
opment of  the  sperms,  254. 

spermatozoids. —  A  name  sometimes  given  to  the  motile  sperm-cells  of 
plants,  271. 

spinal  cord. —  The  part  of  the  central  nervous  system  of  vertebrates  extend- 
ing through  the  spinal  column,  193. 

spinal  nerves,  194. 

spindle,  86. 

spiracles. —  Openings  of  gill  chambers,  as  in  tadpoles;  also  breathing  pores 
of  insects. 

spiral  cells. —  Cells  of  a  fibrovascular  bundle  with  their  inner  wall  thickened 
to  form  a  spiral  thread,  106. 

spireme  (Gr.  spirema  =  a  coil) . —  A  name  applied  to  the  chromatiu  when 
it  forms  a  thread,  prior  to  division,  85. 


420  BIOLOGY 

Spirogyra,  30. 

splanchnic. —  Pertaining  to  the  viscera. 

spleen. —  A  good-sized  organ    lying  among   the    folds  of  the  intestine, 

187. 

spontaneity. —  Power  of  producing  movements  from  internal  causes,  2. 
spontaneous  generation. —  The  theory  that  life  can  arise  in  some  other 

way  than  from  previously  existing  life;  abiogenesis,  10. 
sporangium  (Gr.  spora  =  a  seed  +  angeion  =  a  receptacle). —  A  sac  within 

which  spores  are  produced,  100,  270. 
spores  (Gr.  spora  =  a  seed). —  Single-celled  reproductive  bodies,  capable 

of  growing  into  a  new  plant  without  fertilization,  16,  59,  79,  81,  239, 

267. 
sporoblast   (Gr.  spora  =  seed  -f-  blaslos  =  a  germ).  —  A  sac  in  which 

sporozoites  are  produced,  242. 
sporophyte    (Gr.    spora  =  seed  +  phyton  =  a  plant). —  The    stage,    in  a 

plant  with  alternation  of  generations,  that  produces  spores,  272,  274. 
Sporozoa,  241. 
sporozoites  (Gr.  spora  =  seed  +  zoon  =  animal). —  Spores  that  result  from 

the  division  of  fused  gametes,  241. 
squamosal  bone,  180. 

stamens. —  The  modified  leaves  of  a  flower  that  produce  pollen,  1 19. 
starch. —  A  carbohydrate    with   the  general  formula  CeHioO.s,  or  some 

multiple  of  this,  8,  129,  134. 
stereome  cells  (Gr.  stereos  =  a  solid). —  Cells  in  the  bark  with  very  thick 

walls,  106. 
stereotropism   (Gr.  stereos  =  a  solid  +  trope  =  a  turning). —  Reaction  to 

solid  objects,  53. 

sterility. —  Unfertility,  or  inability  to  produce  offspring,  or  hybrids,  268. 
sterilize. —  To  treat  an  object  so  as  to  destroy  all  living  things  in  it,  14, 

17. 

sternum,  182. 
stigma. —  The  roughened  surface  on  the  end  of  a  style,  for  the  reception  of 

pollen,  120. 
stimulus. —  Any  force  applied  to  an  organism  which  will  produce  a  reaction, 

43. 

stinging  cells,  143. 
stipules,  114. 
stomach,  186. 
stomata  (Gr.  stoma  =  mouth). —  Openings  through  the  epidermis  of  plants 

through  which  gas  enters  and  moisture  evaporates,  116. 
stomodaeum,  284. 
strawberry  plant,  reproduction  of,  244. 


CtLOSSARY-lNDEX  421 

streaming  of  protoplasm. —  The  circulating  motion  of  protoplasm  within 
a  cell,  32. 

struggle  for  existence,  353. 

style.—  The  projection  on  top  of  an  ovary  in  a  flower,  120. 

subspecies. —  A  subdivision  of  a  species;  sometimes  called  a  variety,  371. 

sugar. —  A  carbohydrate  with  the  general  formula  C6Hi2OG  (monosac- 
charide)  or  Ci2H22On  (disaccharide),  8,  132,  134. 

summer  eggs. —  Eggs  produced  in  the  summer  which  develop  at  once,  247. 

sundew,  224. 

sunlight,  131. 

supination. —  Position  of  the  forearm  when  the  palm  of  the  hand  is  upper- 
most. 

support,  139. 

supraoccipital  bones,  180. 

survival  of  the  fittest. —  Same  as  natural  selection,  353. 

suspensorium,  180. 

suture. —  A  jagged  union  between  two  bones,  as  in  the  skull. 

swarm  spores. —  Spores  with  flagella  enabling  them  to  swim;  zoospores. 
74. 

symbiosis  (Gr.  sun  =  together  +  bios  =  life) . —  The  living  together  of 
two  organisms  in  close  relations,  which  may  be  advantageous  or  dis- 
advantageous to  each,  228. 

sympathetic  system. —  Two  chains  of  nerve  ganglia  and  nerves  lying  in  the 
body  cavity,  parallel  to  the  spinal  cord,  195. 

symphysis. —  A  union  of  two  bones  in  the  median  line  of  the  body. 

syncytium  (Gr.  sun  =  together  -f-  cytos  =  cell). —  A  mass  of  living  pro- 
toplasm with  many  nuclei  but  no  cell  boundaries;  acellular,  89,  99. 

synovial  glands. —  Glands  which  secrete  lubricating  fluid  into  joints,  184. 

synthesis  (Gr.  sun  =  together  +  tithenai  =  to  place). —  The  building  of  a 
compound  out  of  simpler  parts,  234. 

systematic  zoology. — The  study  of  organisms  which  gives  attention  to 
classification  and  naming  of  species. 

systemic  circulation. —  That  part  of  the  circulation  which  includes  the 
vessels  that  supply  all  the  body  except  the  lungs,  191. 

systole  (Gr.  systole  =  contraction). — The  period  of  contraction  of  the  heart, 
188. 

tactile. —  Pertaining  to  touch. 

tadpole,  288. 

tarsals,  182. 

taste,  198. 

taxonomy  (Gr.  taxis  =  arrangement  +  nemein  =  to  arrange). —  The  study 
of  the  classification  of  organisms,  19. 


422  BIOLOGY 

telophase. —  The  last  stage  of  karyokinesis,  87. 

tendons. —  Bands  of  connective  tissue  binding  muscles  to  bones,  185. 

tentacles. — •  Appendages  from  an  animal,  usually  motile,  and  serving  as 

sensory  and  prehensile  organs,  141. 
testis. —  See  spermary. 
thalamencephalon. —  The  small  section  of  the  brain  behind  the  cerebrum, 

with  the  pineal  gland  on  top  of  it,  193. 
thallophyte   (Gr.  thallos  =  a  shoot  +  phyton  =  a  plant). —  A  plant  that 

does  not  show  differentiation  into  root,  stem,  and  leaf,  37.5. 
thallus  (Gr.  thallos  =  a  shoot).—  A  flat  leaf  or  branch, 
thermotropism  (Gr.  thermos  =  heat  -f-  trope  =  a  turning) . —  Reaction  to 

temperature,  57. 
thigmotropism  (Gr.  thigma  =  touch  +  trope  =  a  turning) . —  Reaction  to 

mechanical  stimulation,  57. 
thoracic  duct. —  The  large  lymph  duct  in  mammals,  carrying  lymph  from 

the  lower  parts  of  the  body  to  the  veins  in  the  neck,  209. 
thymus. —  A  ductless  gland  in  the  neck,  especially  prominent  in  the  young, 
thyroid  gland. —  A  ductless  gland  in  the  neck  below  the  larynx, 
tibia,  182. 

tissue  (Lat.  texere  =  to  weave). —  A  collection  of  similar  cells  in  a  multi- 
cellular  animal,  27,  95. 
tongue,  185. 

tonsils. —  Two  ductless  glands  in  the  back  part  of  the  mouth. ' 
touch,  198. 

touch  corpuscles. —  End  organs  of  touch, 
trachea  (Gr.  trachea  =  windpipe). —  The  air  passage  from  the  lungs;  the 

windpipe,  209. 
tracheids. —  The  thick-walled  wood  cells  with  tapering  ends,  found  in  fibro- 

vascular  bundles,  105. 
transformation  of  energy,  296. 

transpiration. —  The  evaporation  of  water  through  the  leaves, 
transverse  processes  (Lat.  trans  =  across  +  vertere  =  to  turn). —  Lateral 

projections  from  the  vertebrae,  177. 
Trichina,  231. 
trichocysts,  61. 

tricuspid  valve. —  The  valve  between  the  right  auricle  and  ventricle, 
trigeminal. —  The  fifth  cerebral  nerve,  supplying  the  sides  of  the  head  with 

sensations,  194. 

trypsin. —  A  ferment  found  in  pancreatic  juice,  digesting  proteids. 
tubercles. —  Knob-like  growths,  usually  indications  of  disease, 
tuberculosis,  231,  232. 
turgor. —  The  pressure  of  liquids  within  the  cells  of  plants. 


GLOSSARY-INDEX  42? 

tympanic  membrane  (Gr.  tympanum  =  a  drum). —  The  membrane  cover- 
ing the  ear  cavity  and  serving  to  collect  the  air  waves,  175,  197. 

tympanum  (Gr.  tympanum  =  a  drum). —  The  cavity  of  the  middle  ear, 
197. 

TYNDALL,  14. 

typhlosole  (Gr.  typhlos  =  blind  +  solen  =  a  tube). —  A  cylindrical  rod  ex- 
tending through  the  intestine  of  the  earthworm,  158. 

typhoid  fever,  232. 

ulna,  182. 

Ulothrix,  93,  136. 

unicellular  organisms. —  Organisms  made  of  single  cells,  or  of  colonies  of 
similar  cells,  each  of  which  can  carry  on  all  the  functions  of  life,  52, 
90. 

unit  characters. —  Characters  that  are  inherited  as  units,  359. 

urea. — -An  excretion  from  animals  containing  the  nitrogen  waste 
(CH4N2O),  210. 

ureter. —  The  duct  carrying  urine  from  the  kidney  to  the  bladder,  199. 

urethra. —  The  duct  carrying  the  urine  from  the  bladder  to  the  exterior, 
199. 

urogenital  organs  (Gr.  ouron  =  urine  +  Lat.  gemtalis  =  genital). — The  ex- 
cretory and  sexual  organs,  which,  in  vertebrates,  are  united,  201. 

urostyle  (Gr.  oura  =  tail  -f-  stylos  =  a  pillar). —  The  single  bone  in  the  frog 
which  represents  the  tail,  177. 

use  and  disuse,  LAMARCK'S  theory  of,  351. 

uterus. — •  A  chamber  in  the  oviduct  where  the  eggs  are  stored,  or  in  mam- 
mals where  the  embryo  develops,  200,  291. 

vacuoles  (Lat.  vacuum  =  a  cavity).— Spaces  inside  the  body  of  cells, 
usually  filled  with  a  clear  liquid,  37. 

vagus. — •  A  branch  of  the  pneumogastric  nerve  extending  to  the  heart. 

valves. —  Membranous  folds  in  the  vessels  or  in  the  heart,  which  allow 
liquid  to  flow  only  in  one  direction,  188. 

variability. —  The  quality  of  showing  variations. 

variations. — •  Slight  differences  between  animals  of  the  same  species,  261, 
327. 

varieties. —  See  subspecies. 

vasa  deferentia  (Lat.  vasa  =  vessel  +  deferens  =  carrying  down). —  The 
ducts  that  carry  sperms  from  the  spermary  to  the  exterior,  164, 
250. 

vasa  efferentia  (Lat.  vasa  =  vessel  +  efferens  =  carrying  to). —  The  ducts 
carrying  sperms  from  the  sperm aries  to  the  kidney  in  the  frog. 

vegetative  functions. —  Those  possessed  by  vegetables  as  well  as  animals, 
Associated  with  alimentation  and  reproduction,  217. 


424  BIOLOGY 

veins. —  Blood  vessels  worrying  blood  to  the  heart,  190;   the  fibrovascular 

bundles  in  a  leaf,  114. 
vent. —  See  anus. 
ventral  side,  155. 
ventral  cord. —  The  nerve  cord  on  the  ventral  side  of  the  body  cavity  of 

the  earthworm  and  some  other  animals,  163,  171. 
ventricle  (Lat.  venter  =  stomach). —  The  lower  chamber  of  the  heart  that 

forces  blood  into  the  arteries,  188. 
venus  sinus. —  A  large  blood  vessel  on  the  dorsal  side  of  the  heart  of  a  frog 

into  which  the  venus  blood  collects  before  passing  into  the  right  auricle, 

188. 

vertebrae. —  The  separate  bones  of  the  backbone  or  spinal  column,  177. 
Vertebrata. —  Animals  which  possess   a  backbone  or  its  equivalent,  176, 

373. 

vesicle. —  A  sac. 

vessel  (Lat.  vasa  =  a  vessel). —  Any  hollow  tube  or  cavity,  105. 
vestigial  organs. —  Functionless  remains  of  organs,  formerly  larger  and 

functional, 
villi. —  Projections  on  the  inside  of  the  intestine  which  serve  to  absorb 

food,  308. 
viola,  372. 

viscera. —  The  organs  of  the  abdominal  cavity,  175. 
vital  energy,  or  vitality,  41,  309,  319. 

vitalistic  theories. —  The  theories  that  regard  life  as  a  distinct  force,  323. 
vitelline  membrane  (Lat.  vitellus  =  yolk). —  A  cell  wall  of  an  ovum,  249. 
vitreous  humor  (Lat.  vitrum  =  glass). —  The  transparent  liquid  back  of 

the  lens  and  filling  the  eyeball,  197. 
viviparous. —  Producing  young  alive,  290. 
vocal  cords. —  The  membranes  in  the  larynx  whose  vibration  produces  the 

voice,  209. 
vomers,  180. 
VON  MOHL,  40. 
Vorticella,  91. 

warm-blooded. —  With  blood  that  always  maintains  an  equable  tempera- 
ture. 

WEISMANN,  328,  355. 
wilts,  232. 
winter  eggs. —  Eggs  of  certain  animals,  designed  to  live  over  winter,  usually 

requiring  fertilization,  in  distinction  from  summer  eggs  which  do  not, 

247. 

Wolffian  body. —  The  primitive  kidney  found  in  the  vertebrate  embryo, 
wood. —  Same  as  xylem. 


GLOSSARY-INDEX  425 

worker  bees. —  Female  bees  whose  sexual  organs  do  not  mature,  and  who 

perform  the  work  of  the  colony, 
xerophytes    (Gr.   xeros  =  dry  -f  phyton  =  a    plant). —  Plants    inhabiting 

very  dry  regions, 
xylem  (Gr.  xylon  =  wood). —  The  layers  of  hard  woody  cells  inside  the 

cambium  layer  in  exogenous  stems;  the  wood,  105. 
yeast,  78,  235. 
yolk. —  The  food  material  deposited  in  an  egg  for  the  nourishment  of  the 

developing  embryo,  250. 
zooid  (Gr.  zoon  =  an  animal) . —  A  more  or  less  independent  member  of  a 

compound  organism,  like  a  hydroid,  148. 
zobspore  (Gr.  zoon  =  an  animal  +  spora) .  —  Spores  which  swim  by  the 

agency  of  motile  flagella  or  cilia,  93,  136. 
Zoothamnium,  92. 

zygapophyses. —  Articular  processes  in  vertebrae,  177. 
zygospore  (Gr.  zygon  =  a  yoke  -f  spora) . —  A  spore  formed  by  the  union 

of  two  gametes,  94,  262. 
zygote  (Gr.  zygon  =  a  yoke). —  A  cell  formed  from  the  union  of  two  other? 

in  sexual  reproduction;  a  zygospore,  94,  262. 
zymogenic. —  Giving  rise  to  fermentations. 


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